You have to have the "unallocated" space directly to the right of the drive that you want to expand. With your picture above, you could expand the D drive with the unallocated space to the right of it.
That is exactly what I want to do, but as my screenshot shows, it does not let me!
However, if you want to expand the C drive...you would have to delete the D drive partition immediately to it's right to do it.
I didn't intend to expand the C partition. Also, it is not adjacent!
The D is where the restore points are kept, and I want to make room for more.
There may be a reason that the system doesn't allow it to be made larger?