Good question! Answer choice D is designed to throw you off as it mentions certain key words from the passage.
Answer B is correct. The passage begins by talking about Lamarck, and tells us (starting roughly at line 7) that he has been ridiculed for his ideas. Then, it moves on to tell us that Steele (line 12) is trying to revive Lamarck's ideas. The rest of the passage tells us how Steele is doing this. Answer B states that the passage is primarily concerned with "describing the efforts of a modern biologist to vindicate a long-disregarded evolutionary theory." Steel is a modern biologist, and he is trying to revive Lamarck's ridiculed theory.
Answer D is incorrect for two reasons. First, the passage never evaluates the overall merits of Lamarck's theory. Rather, it simply tells us a bit about his theory in the beginning and tells us that he was ridiculed. Second, this answer choice fails to take into account the role of Steele in the passage. The passage describes Lamarck's ideas at the beginning, then moves on to describing Steele's work, while this answer choice only focuses on Lamarck.
