The main clause is "Chris's chief interest was tennis".
The subordinate clause is "Which he was just learning to play" and it is an adjective clause because it describes what the "tennis" was like. Tennis is an object and this subordinate clause further describes that it is the tennis that he was trying to learn.
I'm not 100% sure, but I think the answer is C - Order of Importance. Hope this helped any!
Is it okay if you add a picture to it?
