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Before enlarging your system partition, make sure there is some unallocated space clinging to the system partition, otherwise it would fail. If there is no unallocated space clinging to your system partition C, move the partition D first to make some space between partition C and partition D.
After that, drag the right boundary of the C: to enlarge it. The last step, apply these operations. You will get a enlarged partition C after reboot.