What do you need 100mb for? It's not a partition so doesn't count against maximum four Primary before you must convert one to Logical to add even more.
If you want it then use the PW Extend function from C, have DVD or Repair CD standing by in case you need to run Startup Repair after resizing on the boot sector which happens in about 10% of these cases.
In Disk Management, you can only extend partition using the unallocated behind the partition to be extended.The Extend Volume option will be greyed out when the unallocated space before C drive. In your case, you can use free edition of AOMEI Partition Assistant to add the unallocated space to C drive.