Windows 10 Unallocated drive space - wont move into C: drive

I want to provide a lot of space for a Linux Mint distribution to run on the windows pc, having first set up a dual boot system. I had a D: drive with 800MB of space not being used. I deleted the D: drive and that space became "Unallocated". Following the instructions, I should be able to click on the C: drive and get an option of "Extend the drive" but that option is "greyed out." So I cannot move the unallocated space into the C drive. There are 2 other allocations (not named as drives) between the C and the D drive (that is now an unallocated space). I cant find any way to move that space into the C drive?? I have re-allocated the space now back into a D: drive and that is fine and shows up as a regular drive - so I'm back to square one! As the aim is to install Linux Mint, and run it when I want to, can I install Linux Mint into the D: drive instead of making a dual boot system? So it would act like any other program that I have installed on my PC ??