Explanation
Correct answer: A.
Choice A is the best answer. “Therefore” logically signals that the information in this sentence—that it was necessary to process the acorns before eating them—is a result or consequence of the previous information about raw acorns having an extremely bitter taste due to high tannin levels.
Why the other choices are wrong
Choice B
Choice B is incorrect because “similarly” illogically signals that the need to process acorns is similar to the previous information about their bitter taste. Instead, the need to process the acorns is a result or consequence of their bitter taste.
Choice C
Choice C is incorrect because “afterward” illogically signals that the need to process acorns is an event that occurred later in a chronological sequence than the acorns’ bitter taste. Instead, the need to process the acorns is a result or consequence of their bitter taste.
Choice D
Choice D is incorrect because “however” illogically signals that the need to process acorns contrasts with the previous information about their bitter taste. Instead, the need to process the acorns is a result or consequence of their bitter taste.