Choose the case 2: Merge unallocated space into one partition. Don't need to create F drive, In the first post, Plutox mentioned:"My C Partition has somehow got split up with a following D backup partition followed by an unallocated partition." Therefore, just delete F drive and make it as unallocated partition again. Then, use the partition manager to merge unallocated space to C drive.
As I understand it, the partitions are:
C D F
If you delete F, that leaves the unallocated space adjacent to D, not C, so it can not be merged into C as it is not adjacent to C.
The partition manager can merge unallocated space (available for adjacent or not adjacent to the partition you want to extend) to a partition .