If "D" is physically on the same hard drive, then you'll probably want to back it up before installing. Technically, you can manage to do it without screwing everything up, but I would not trust myself to. It's always a good idea to have backups anyway.

Also, user files typically reside on C by default and it takes some effort to put them on a different drive. Things like Downloads, Documents, Pictures, etc. so it's worth checking that before wiping as well.

Additionally, you'll probably want to format your "D" drive to a Linux native filesystem (eventually, after you back it up, because formatting results in data loss). While Linux does support NTFS quite well, it's not perfect, and your data would probably be safer on ext4 or f2fs (depending on if you have HDDs or SSDs) (or zfs or btrfs is you're into COW filesystems).

In Linux, you have all of your files mounted to a single "drive" called /. Everything is below /, which is called the "root" of your filesystem.

Typically, user data is stored in "/home" and this resides in the same directory structure as the rest of your OS, but on most systems it's on a different filesystem or even on a different drive entirely. This is because in Linux it is routine to put a "D" drive just in a folder. On my computer, I have several of these mount points defined, so the different types of data don't get mixed around, and I don't have to worry about downloading too much bullshit affecting my computer's updates.

Hope this helps.