It's perfectly simple. Replace 'chorus' with 'soundtrack'. It's a 'knowledge of dramatic technique' question. Examine what would be lacking in the play (and in the audiences' comprehension thereof) if the chorus wasn't there.
I don't know Oedipus Rex as a play, but I can spot that type of question a mile off.
Unrelated to
Curlyclassicistgirl's Greek travails, or to the elbow-bashing discussion: a question for my /RB/-ish brethren (and sistren):
Given that the several-score channels on the 'lectric fishtank are all filled with mind-numbing drivel, to what fine cinematic work should
Eggy direct his attention this evening?