The answer is C. The writer's attitude towards the audience and subject.
Eclipsed .... like when you have a solar or lunar eclipse
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.
