Why do you think you need a larger C partition? Just be aware that re-sizing partitions can trash the entire thing and you may end up losing all your data on both partitions. If you must go down that road you need to first reduce the size of the D partition before expanding the C into the freed up space.
What are you doing wrong with your file & data management that 100GB isn't enough of a C drive? Manage your data differently and you do not need to risk messing with partition re-sizing.
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