Hi. I have no experience in C, but I know C++ quite well. Would it be much of a challenge to learn C using raylib or SDL (I have used both of those libraries in C++) ? I dont want it to be too easy whilst being manageable. How else could I challenge myself?

Have you used the SFML for c++? if not then that's a good choice, They told to choose a different game engine/framework not a completely different language