You can move all the data under your " Users " folder to free some space on your C: drive. but other files like windows files and program files should remain on your C: drive to avoid conflicts.
You can also change the file directory of your download folders and the files that you want to save to your D: drive for you to save storage.
Open the Settings app.
Click on System and then on "Storage" from the side-bar on the left.
Scroll down to the bottom of the page, where it says "More Storage Settings".
Click on the text that reads "Change where new content is saved".
You are taken to a new screen which displays the default save locations for six different types of data: Apps, Documents, Music, Photos and Videos, Movies and TV Shows, and Maps.