If you want to enlarge C with Disk Management,
1. Save everything from D to an external drive.
2. Then right click on D and delete the volume to get unallocated space.
3. Right click on C and extend it to at least 50GB
4. Right click on the remaining unallocated space and define a new D partition
5. Move the saved data back to D
That is the easiest way to do it. Else you will need another program like Partition Wizard
. But even then I would first backup the data on D for safety reasons.