How can I tell the Dropbox desktop app to look at the D drive? And is it safe to delete C:\Users\PC\Dropbox?
Save yourself some time and frustration trying to move things around after it's installed like this.
Instead, uninstall Dropbox if it's installed. Rename your existing Dropbox folder on your D drive to Dropbox_OLD or similar. I would rename the one on C drive as well, just to be safe. Reinstall Dropbox and select the location that you want for the Dropbox folder. There's usually an advanced options section after you sign in that allows you to do this. If you want it to be D:\Dropbox, then just select the D:\ drive so a new Dropbox folder is created. When the installation is finished, Dropbox will immediately start to sync. Exit the Dropbox application so syncing stops. Move the content of Dropbox_OLD into the newly created Dropbox folder. When the move is complete, and not before, resume syncing or re-launch Dropbox.
At this point Dropbox will begin indexing all of your files. This process will take a while, especially if you have a lot of data to go through. During this time it may say that files are uploading or downloading, but it's only transferring comparison data and any changes that it finds. Be patient and LET IT WORK.