Answer: B. They fear for their lives.
In Act III, scene ii, we see Stephano, Trinculo and Caliban plotting against Prospero. Caliban hates Prospero and desires to be free, therefore, he is ready to join Stephano in his plans to take over the island and marry Miranda. As the group talks, they drink, and the scene is comical as they begin to get more and more intoxicated. Ariel appears, and he is not happy with the scene. After interferring verbally, he plays music. Upon hearing the music, the men, who cannot see Ariel or the source of the music, become very fearful.