Explanation
Correct answer: B.
Choice B is the best answer. “First” logically signals that the question in this sentence—whether there is a daily period during which jellyfish pulse rates decline—is the first in a sequence of three questions the researchers attempted to answer about jellyfish sleep behavior.
Why the other choices are wrong
Choice A
Choice A is incorrect because “as a result” illogically signals that the question in this sentence is a result of the three questions the researchers attempted to answer. Instead, it is the first of those three questions.
Choice C
Choice C is incorrect because “additionally” illogically signals that the question in this sentence is an additional question related to the three questions the researchers attempted to answer. Instead, it is the first of those three questions.
Choice D
Choice D is incorrect because “however” illogically signals that the question in this sentence contrasts with the three questions the researchers attempted to answer. Instead, it is the first of those three questions.