Hi Yve,
I'm Sumit, here to answer your query at the Microsoft Community.
To expand C drive, you would need a 3rd party tool to get free space from Drive D.
Minitool free version should do that for you:
https://www.partitionwizard.com/help/resize-par...
Disclaimer:
The link included is a non-Microsoft website that would provide accurate and safe information. Watch out for ads on the site that are frequently classified as PUP (Potentially Unwanted Products). There is no need to buy paid products to fix your computers as they sometimes do more harm than good.