If you have two separate drives, take your choice from the following. What OS are you running?
2) Backup everything on them, wipe them, put them in RAID 0, restore them.
3) There is a little-used (for good reason) ability in Disk Manager to mount one drive in a directory on the other. But that doesn't increase the free space on either, it just offloads one directory worth of stuff from one to the other.
4) Buy a bigger drive and copy everything to it.
OHH - I just read your post again. Do you have ONE drive with TWO partitions? Everything above applies to two drives.
With two partitions on one drive, assuming that they are right next to each other, you can simply erase one partition and expand the other into it. Do backups before trying any of this, just in case.
Assuming that you have no OS on D:
If all the data on D will fit on C, it's easy. Just copy the data from D to C, verify that it is really copied, and open the Disk Manager. Delete partition D, and then expand partition C.
If all the data on D will not fit in the freespace on C, then the above won't work. If you have good, reliable external backups that you trust, you can drop partition D (erasing all the data), expand C into the space, and restore your backups.
A third possibility that I wouldn't trust unless I had good backups: Use some clever utility that will allow you to "merge" the partitions, preserving the data. I'll give you 3-to-1 that it would work, with a one in four chance that you will lose everything.