Traditional way of merge D Drive to C Drive in Windows 7 is to delete D Drive and make the space unallocated, then extend C Drive to with the space from D Drive. Actually, it's not real merge, just extend volume on C Drive, because data in D Drive is lost.
Anyway, if data on that partition is not important, follow me do the traditional way of merging.
Or, check the screenshot guide on how to extend volume in Disk Management.
That's the traditional way, or the misused way, of merge partition
Other third party disk partition manager, however, didn't correct the misunderstanding, but keep on deleting Drives and extend the others. That's simply meaningless of copy disk management. Merge partition on software should keep data in the original partition intact or stay as they were.
If you want to merge D Drive to C Drive without data loss, and keep both volume labels please read on.
Partition Extend will merge D Drive to C Drive within one click and without data loss.
In this way, you keep data on D Drive safe and you have all the free space on D Drive merged to C Drive without data loss on both Drives.