This question is essentially asking us for the author's feeling toward the different explanations that are presented throughout the passage. We do know that the author believes that the rate of speciation hypothesis is the most plausible (line 42) and that all of the other theories have weaknesses.
(B) says, "The rate of speciation hypothesis addresses a principal objection to the climatic stability hypothesis."
(B) closely aligns with the author's attitude about the rate of speciation hypothesis vs. the other ones. The principal objection of the climatic-stability hypothesis is that it doesn't account for the regional pools of species. Regional pools of species need to be discussed in order for the theory to provide a more comprehensive understanding (this discussion occurs in lines 34-41). In the second to last paragraph, the author states that the fourth and most plausible hypothesis (rate of speciation hypothesis) focuses on regional speciation, so this tells us that it is addressing a principle objection (the lack of focus on regional speciation) to the climatic stability hypothesis. Therefore, (B) is the correct answer.
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