The question says, "Based on the description in the passage, a poem that reveals gender differences in the absence of any specific intention by the poet to do so is most like..."
(D) says, "a railroad track, without which travel by train is impossible"
The problem with (D) is that it deals with a scenario in which something is required in order to do something, but what we're looking for is some sort of underlying trait. (D) does not reveal anything to us, so it's out.
(A) says, "a bird’s flight that exposes unseen air currents"
The passage tells us that gender differences will eventually become manifested involuntarily in poetry when the poet isn't intentionally trying to show it. We're looking for an answer choice that reveals some sort of underlying characteristic in a simple manner as a byproduct of its normal functioning. We know that a flying bird isn't attempting to expose unseen air currents; rather, it exposes them by acting as it normally does. Also, since we know that air currents are an underlying component of flying, (A)'s analogy works well, as the air currents map over to gender and the flight maps over to a poem. Thus, (A) is correct.
Does this make sense? Let us know if you have any more questions!