In comparison to people, elephants are capable of perceiving a much wider variety of sounds; in fact, an elephant’s ears can pick up frequencies more than 20 Hertz lower than what human ears can hear.
Meaning: Elephants can perceive a much wider variety of sounds compared to humans. An elephant's ears can pick up frequencies more than 20 Hertz lower than those [frequencies] human ears can perceive.
A) lower than
what human ears can hear.
Grammatically correct. I have kept option A for comparison with option D.
B) lower than those
by human ears
by is not appropriate in the context. The use of by makes human ears a prepositional phrase that illogically modifies those [frequencies]. It suggests that human ears produce frequencies, which is illogical. Eliminate B.
C) lower
in opposition to the range that human ears have.
in opposition is unidiomatic and inappropriate in the context of comparison. We are dealing with similar abilities, hence there is no need to use words that show contrast. Eliminate option C.
D) lower than
those perceptible to human ears
This option is also correct grammatically. Option D is compared with option A below.
E) lower
as opposed to humans’ hearing
Similar to the issue of wrongful use of contrasting words when similarities between two entities are compared. Eliminate option E.
Now my headache was choosing between A and D. But looking at the meaning, option A: what humans can hear, means any frequency that humans can hear. It can be loud, medium, or it can be low. But the context requires a comparison between minimum frequency that human ears can hear and that an elephant's ear can perceive, of which we are informed that elephant's ear can hear frequencies 20Hz lower than what humans can hear. Based on this option A can be eliminated as a result of the ambiguity in meaning.
Option D on the other hand, those perceptible to human ears, means that frequencies that humans can barely hear. So it limits the frequency to low ranges and this makes more sense with the overall meaning of the sentence.
The right answer is option D.The bottom line for me is that meaning is always key.