Space used by a single drive letter has to be contiguous. You cannot "bridge" across another drive partition to connect to unallocated space.
In the lower Disk manager view how is the drive space allocated? Before the C drive allocation you may have a small (100MB) System reserved, then hopefully the C Drive if that is not the 1st area. In order to extend the drive the next block to the right needs to be the black unallocated space. If it is not then you cannot extend the C drive.