If there's no other partition to transfer files, or if D drive is Logical, you cannot extend C drive even if deleting D. Obviously, it's not a good idea to extend C drive by deleting other partition.

Right click D: and select Delete Volume, its space will be changed to Unallocated.Press Windows + X together on the keyboard, click Disk Management in the list.Back up or transfer all files in the adjacent D: drive.Steps to extend C drive in Windows 10 using Disk Management tool: D drive in this article means the contiguous partition on the right of C drive.Do not delete D if you installed programs in it.If you want to increase C drive space for Windows 10 computer without any software, you must delete D to get contiguous Unallocated space.