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 agree with the person above - the correct answer should be C. the use of regional dialect.
A dialect is a type of speech which differs from the accepted, grammatically correct, standard English. Here are some examples from this novel which portray the way she speaks: “You answer me when Ah speak. Don’t you set dere <span>poutin’ wid me after all Ah done went through for you!”</span>