Answer:
A is the correct in my opnion.
It's pronounced "ensemble" by the way. And, one way that a pianist would have a role would be by being in the background as an example and playing his part. He's going to play what he needs to play. The question does seem a little to vague, but truly, I would say simply, that the person who is playing it, just plays his part as they all make music sound good.
