Answer: B. It provides a humorous parallel to the main plot.
Step-by-step explanation:
Shakespeare's The Tempest explores the theme of subordination embodied primarily in the relationship between Caliban and his master, Prospero. The subplot involving Stephano and Trinculo includes some of the most important issues present in the main plot, but serves as a comic relief from the main action. For example, Stephano and Trinculo joke about selling Caliban for a piece of silver, calling him 'a strange fish' (Act II, Scene II).