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#11
You cannot add the unallocated to C with Windows Disk Management.
Why? Because in your case, Partition is between unallocated and C. Windows Disk Management is inflexible and allows adding ONLY if there is nothing in between.
If that picture read C, then unallocated, then Partition, you could add unallocated to C.
So you must use Partition Wizard to add unallocated to C. It's flexible in that way and give you much more control.