if they are just partitions on the same HD then you can use tools such as achronis disk director or paragon partition manager to delete drive D and merge it into drive C. if they are 2 separate HD's then you cant with out using raid.
Both my C and D drives have 111gb storage. D drive is totally empty and my C drive is totally full. How do I reduce size of D drive and increase the size of C drive? Thank you.
What's your OS ? If your OS is Windows 7, Vista or 2008, you could do what "ncc74656" said but without other third-party software, the built-in disk management offers you the function "Extend volum" but if you do not want to delete the d drive or you were not running the Windows 7, VISTA or 2008, you could only do that by useing some third party software, such as disk director, partition assistant, Gpard, Partition Master, and Partition Wizard.
Here is a resource about "Solution to Windows 7 and Vista Problem: C Drive Full, D Drive Empty"