If you have the C partition with D partition next and then the unallocated space. When you try to try to extend the C drive you cannot due to no space available at the side.
To extend the C drive.
Click and hold mouse button on D partition and drag it to the right you will now find the unallocated space between the C and D partition.
Click on C drive, go to move/resize partition. You can now use the unallocated space.
Hope this helps.