No need to convert D, just boot free Partition Wizard bootable CD, rightclick D to REsize, grab the D partition with mouse and drag it to the right to take up the Free Space.
This will create Unallocated or Free space to its left allowing you to rightclick C to Resize by dragging the right grey border over it - the latest versions of PW view Unallocated (primary) and Free (Logical) space interchangeably.