In the context of the story, the reason Don John gives for his sullenness is B) He figures that if he throws a fit, he'll get his way. Don John is portrayed as a manipulative and deceitful character who seeks to cause trouble and disrupt the happiness of others. By pretending to be sullen and unhappy, he believes that he can manipulate others into giving him what he wants. This reason aligns with his overall character and motives in the story, as he consistently engages in deceptive tactics to achieve his own selfish desires.
It is important to note that this answer is specific to the context of the story being referred to. Different stories or situations may present different reasons for a character's sullenness, so it is essential to consider the specific details and circumstances of the text when analyzing a character's motivations.