Well, as I've already mentioned it's all up to personal preference. I guess you're going to try with a 3rd party partitioning program, as it sounds like the C and D partitions are not next to each other and this won't allow you to extend the C drive with space, which is not right next to it on the right side if you're using Disk Management. Just make sure that you backup any valuable data which you might have, so that you're on the safe side in case something unexpected happens.
Cheers!