Why was the chorus so important in an ancient greek play?

The chorus in an Ancient Greek play was extremely important because it helped the viewers to follow along. Often times it summarized what happened in a certain act/scene/play, which helped people who did not understand it to, well, understand it. It also often provided insight into characters minds. It told things about the characters that they could not tell about themselves, which could help the viewer predict what was going to happen in the upcoming scenes.

Not b or c so that leaves you with a or d. a is the better choice.