You could delete some files and folders on your C drive to make more room, but if you are saying that you want to take some memory from the D drive over to the C drive, that is not possible.

The drives are fixed sizes, and cannot be transfered to one another, unless of course D is just a partition of the C drive, in which case you can increase and decrease the size of both using a disk management utility but you might lose some data.

Also, you could make your D drive your C drive, and vice versa, but then you would have to transfer everything over from your C drive to your D drive at first, then change it to the C drive.

The only way to change your D drive to your C drive is to open up the computer, and switch the drives positions and connections. If you want to learn how to backup your entire C drive to the other D drive, then check out this page on data backup solutions.