Answer: The purpose of the idiom in the statement is C. to show that Jen is in a difficult position.
Explanation: "To be between a rock and a hard place" is an idiomatic expression, that is to say it is a phrase which has a figurative meaning that native speakers understand. In that way, the purpose of the author is not to indicate that Jen is literally found between a rock and a hard place, but to show that Jen has a dilemma or is in a very difficult situation which forces her to make a hard decision.
