As far as looking to see what's on each of your partitions, you can just use Windows Explorer to browse them. Your screenshot shows that each partition has a drive letter, so Windows Explorer will show you what's there.
Your current 100MB "D" partition is what appears to have at one time been your "system reserved" partition, which at that time would also have been marked "active" in the upper pane information. It's where Boot Manager was once living and functional, and when you booted the machine the BIOS found the "active" partition and kicked off Boot Manager to handle the rest of the operating system loading and initiation. If you had two or more bootable OS's you would have been presented with a Boot Manager Menu of choices, for you to pick from to boot that OS. If you only had one Windows, then Boot Manager would have gone directly to that single Windows to boot.
Normally the "system reserved" and "active" partition only contains Boot Manager and is NOT intended for user data storage. So Windows does NOT assign a drive letter to it.
However in your screenshot, it appears that this 100MB partition is no longer marked as "active". Instead, it appears your C partition (where Windows actually lives) is now the "active" partition, hence why the small 100MB seemed eligible for drive lettering and user storage... hence why it now shows as D.
This is unusual and non-standard, but I'm guessing at one time you may have used Windows Repair to recover some unbootable problem situation (perhaps when you tried to shrink your original C which took up most of the drive, in order for you to create a "data" partition which currently shows as E), and it was that "repair" which actually installed Boot Manager into the C partition and changed things to mark C as "active" (so that the BIOS would go to that partition to find Boot Manager at machine boot time). Hence why the 100MB original "system reserved" (and where Boot Manager also is probably still living, though no longer used from here) was deemed user-available and was lettered as D, though obviously you can't put much in that small a partition of which only 86MB is actually free right now. You really don't need this partition at all.
I'm guessing your 4GB F partition (of which most of it is free) was probably delivered as some kind of "recovery" partition by Dell, if you ever wanted to for some reason restore things to "factory" initial version. I've never found any such capability of any use, as I prefer to use Macrium Reflect to make "system image" backups (to an external USB 3.0 drive) as my recovery capability in case of disaster. I can make an initial "gold/factory" image to start, before doing any of my own customization. And then I can take regular periodic "system image" backups of my ongoing live customized "production" system, as my own protection against who knows what type of problem that might render Windows or the system unusable or unbootable.
In any case, I'll suggest you can merge that last 4GB partition into your E "data" partition to its left and just have E.
I'd suggest using
Minitool's Partition Wizard to at least do some housekeeping. Since D is worthless now (since C is the "active" partition), you can delete D and shift C all the way to the left to absorb that 100MB. You should burn the standalone bootable CD from the ISO available on the Minitool site, and boot to it to do the operations affecting C.
Also, you can delete F and then expand E to the right to absorb that 4GB into E. You can do this at the same time you are standalone booted to Partition Wizard to work on C, or if you want you can use the Windows runnable version of the program, since E and F don't involve C and thus the normal Windows program version is directly usable without concern.
So you'll lose drive letters D and F. If you want to retain E as E (thus now giving you C and E), that's fine. Or, if you prefer you can use DISKMGMT.MSC to change the letter of E to D, if that is more comfortable for you (so that you then have C and D).
Quite frankly, I would recommend that you use your "data" partition (E now, or perhaps D if you change things as I suggest) for DATA... and not for programs. If you feel you're tight on space on C, you can use Partition Wizard (standalone booted) to (1) move the left end of D to the right, thus shrinking it somewhat, and (2) move the right end of C to the right, thus enlarging it somewhat. Partition Wizard will take care of all of these operations for you, to resize and relocate partitions and their boundaries as you desire.
You can store documents and files of any type (text, DOC, XLS, JPG, MPG, etc.) anywhere... either on C or D, but if you want to reorganize your thinking to use C for programs and Windows and D for "data", that might make it easier in the future to keep track of and backup/restore your data as becomes necessary. Having a good backup regimen for both "data" as well as "system image" (both of which should go to external media or a second internal hard drive) is vital to guaranteeing that you will never lose any of your priceless irreplaceable data, nor will you lose more than 20 minutes tops to get your Windows operating environment restored in the event of a software or hardware disaster, or if you want to upgrade your hardware.
You would just create a set of several new high-level folders for "data" on D. Then you would gradually create a satisfactory set of sub-folders in those "parent" folders, to keep your data organized as you want. You can use Windows Explorer (or other Explorer replacements that are much niftier to use, such as
Free Commander) to MOVE your existing data out of the standard \My Documents locations on C, into your new "data" folder/sub-folder structure on D. Going forward, any time you are offered a choice to "Save as...", just navigate to your new data folder structure on D and don't save it on C.
Macrium Reflect (free) is highly recommended for "system image" backup, and the non-free product version can also be used for folder/file "data" backups as well as providing some feature enhancements for "system image" backups. Many other similar products are also available for you to try and decide for yourself. I myself use Macrium Reflect for "system image" and
NovaSTOR's NovaBackup (not free) for "data" (just because I prefer its GUI and functionality).
What's important is that everybody should be taking whatever backups they need to guarantee that they do not "cry" in case of software or hardware disaster, because they've now lost irreplaceable data or don't know how to reinstall Windows and all of their 3rd-party vendor software.