Can't extend C partition. How to make unallocated space "adjacent"

You cannot do that with windows onboard tools.

There are various tools available that would allow you to move the D partition to the end of the free space, thereby freeing space after C, but this is a relatively dangerous operation (if you have a power failure during the move, you could loose the content of D or the whole disk), and not strictly necessary.

In your case, it's probably easier to just

• create a new partition in the free space
• move everything from D to there (turn of the page file first, and activate it again at the end of the whole operation!)
• delete D and relabel the new partition to D
• After that, you can expand C into the free space (and also the new partition, if you want it to be as large as D originally was).

(NB: on a hard disk, the space for a partition needs to be physically contiguous, on SSD's, this should be irrelevant, but that's what you get when you insist on emulating old hardware...)

Slightly Left field answer:

1. Create an E: drive in the currently unpartitioned space.
2. Transfer your files and folders from D: to E:
3. Delete the D drive freeing up that space.
4. Rename E: to D:
5. Extend the C: partition into the empty space now adjacent to the partition.

I'm not sure how effective this solution would be for someone with more data than you have. 30GB shouldn't take too long to move.