Easeus Partition Master Home Edition 6.0.1
Merging partitions is not recommended to increase c drive since merging partitions is a danger to your data and system. If you insist on merging partitions, do backup your system and other partitions before merging. However there are more smart way to do the trick.
In fact you can shrink your partition which has a lot of free space to make unallocated space, and move the unallocated space to the c drive by moving the partitions in between. And then extend the c drive.
For Windows desktop uses, here is a tutorial with Acronis Disk Director Suite http://www.geeksdo.com/extend-c-drive-pc-acronis.html
(Actually this program can let you merge your partitions to increase size c. However as I said, it is not recommended to merge partitions with any software.)