Thanks for your replies.
I've been toying with these programs for a while and I think I've found a solution, but I want to run it by you all first.
As shown by the picture in my original post: Between the unallocated 16 GB and the main C: partition is the EFI System Partition, which holds the BIOS for my computer. If I delete that, my computer of course won't boot. And neither EaseUS nor Partition Magic will let me move the EFI System Partition to the left of the unallocated space (so that I'd be able to extend the unallocated space into C:, solving my problem).
Instead of deleting the EFI System Partition, I'm thinking I could copy it into the unallocated space, then delete the old EFI System Partition. At that point I'd have, from left to right, "EFI System Partition," "Unallocated Space (16GB)," and "C:". And then I could merge the unallocated space with C: since they'd be adjacent.
But this seems risky to me. Since I don't know if the copied EFI System Partition (which, again, carries the BIOS) would be in some way different from the original EFI System Partition (which I would have deleted), it seems possible that the BIOS would stop working if I carried this out, rendering my computer inoperable.
So, does anybody know how I can safely "move" (copy and delete) the EFI System Partition without breaking my computer?