Explanation

An initial reading of this sentence might suggest that the blank should be filled with a word like “complex” that indicates how hard it is to “make sense of” the digressions. However, there is no such word among the answer choices. Focusing on the second half of the sentence suggests a different interpretation. According to the sentence, it would be “wise to” make sense of the digressions, and a textbook would help the reader to do so. If the digressions are “uninteresting,” “unsophisticated,” or “humorless,” the sentence provides no reason to think it would be wise to make sense of them, and if they are “controversial,” it provides no reason to think that a textbook would help. Only if the digressions are “frustrating” does the sentence make a coherent whole.
Thus, the correct answer is frustrating (Choice D).
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