Concept:The phrase “cocked their ears” is an idiom that means listening very carefully and attentively.Explanation:The expression “cocked their ears” suggests the action of an animal turning its ears toward a sound to hear it clearly.This idiom does not mean lack of interest, lack of understanding, or a casual attitude.It is used when someone listens with full concentration and close attention.Therefore, the students listened with rapt attention, or complete focus.Answer:B. rapt attention