With Partition Wizard (link above) you can move D all the way to the right. Then you can combine the 2 free spaces and add them to C. Here is a tutorial on Partition Wizard: Partition Wizard : Use the Bootable CD
I would also suggest to make a backup of D before moving it. You never know what might happen. Usually nothing - but on a bad day?
Addition: If you want to do the move with disk management, that is a bit more complicated. You can backup or image the D partition. Then delete it and consolidate the 3 free spaces. Then add part of that free space (less the size of the original D) to the C partition. Then recreate the D partition in the remainder of the free space and move your data from the backup into it.