Explanation

Recycle the clue that Sheila made what seemed like bitter complaints. None of to waffle, dissemble, or mince mean to make bitter complaints, so eliminate choices (A), (C), and (E). You might have associated grieve with the word grievance, but it actually means to mourn, so eliminate choice (D). To rail is to make bitter complaints, so select choice (B).

Hence option B is correct!
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Confused with the answer choice B. Need more accurate explanation.
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prompt is a contrast as BUT is there after comma. Hence both parts of sentence are opposite in meaning to each other. In second part of the sentence it is said that people believed that Sheila complaints were facetious ( not serious). hence we can actually use word BITTER in first sentence also as it will convey exact opposite meaning to the first part of sentence. Now we can easily search the options which resembles Bitter. RAIL is the perfect answer

Shouldn't it be rail against then.

here Rail is being used as verb. so rail about or rail at are proper usage