You can only expand a volume into adjacant space. So you would have to move D to the end of the now unallocated space it consumed before. Use another partition manager if yours doesn't do that.
It's unclear why you couldn't delete it. Maybe another partition manager can? Try GPARTED from a Linux distro.
I was going to delete partition D and then expand C to use the extra volume. When I right clicked on D, delete volume was not highlighted to I shrank the volume to 100,000 +. I then right clicked C but the expand volume is not highlighted. What can I do. I did not want to move the files to D.

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