who is used to ref to Human Being only. & of which vibrate to cause roar is not logical in E.

its between A & B.
its is wrongly used in option A>

B is best fit.

+1 for B. IMO, C was close but the word 'differently' is not reqd.

I don't think you can use 'who' to refer to animals.

We can eliminate c and d because of "who" which cannot be used for animals.

"Of which the hvoid bones.." in E does not make sense, as you need a word that refers to lions and tigers (=which).

Thus we need to chose between A and B.

B does not makes sense as the animals "can roar" and not "can be roaring". If you are unsure between A and B you can look at the end of the sentence. You need the same structure here.

OFFICIAL EXPLANATION

The original sentence incorrectly says "can be roaring", when the appropriate present-tense verb form is "can roar". The sentence also makes the mistake of using a singular possessive pronoun ("its") to refer to a plural antecedent ("lions and tigers").

(A) This choice is incorrect as it repeats the original sentence.

(B) CORRECT. This choice corrects the verb problem in the original sentence by replacing "can be roaring" with "can roar". This choice corrects the pronoun problem by using the plural possessive pronoun "their" instead of "its".

(C) This choice incorrectly uses "who", instead of "which", to refer to lions and tigers. On the GMAT the pronoun "who" is reserved for human beings; animals and things are referred to using "which". Another mistake in this answer choice is the inclusion of the redundant word "differently".

(D) This choice makes the mistake of using a singular possessive pronoun ("its") to refer to a plural antecedent ("lions and tigers").

(E) This answer choice illogically makes it seem as if the hyoid bones of lions and tigers vibrate, and thereby create roaring sounds, independently of whether the lion or tiger actually wants to roar. The use of the singular "a roar" is also inappropriate, because it appears to suggest that many lions and tigers collectively emit just one roar.
_________________

Unlike lions and tigers, which can be roaring by causing its hyoid bones to vibrate, domestic cats have fixed hyoid bones and are therefore unable to roar.


A. which can be roaring by causing its hyoid bones to vibrate, domestic cats

B. which can roar by causing their hyoid bones to vibrate, domestic cats

C. who can roar by causing their hyoid bones to vibrate, domestic cats differently

D. who can roar by causing its hyoid bones to vibrate, domestic cats

E. of which the hyoid bones vibrate to cause a roar, domestic cats

Is the ", which" referring to both the lions and tigers, or just the tigers? My understanding of the use of which is that it can only refer to a single noun. Can which refer to 1 or multiple nouns and if so, is the ", which" in this sentence referring to the whole clause "Unlike lions and tigers"?

Immediately “which can be roaring” should strike you as incorrect.

A is out.

Sentences that start with “who” are also incorrect. Lions and tigers are not people, and ‘who’ is used to refer to people. 

C&D are out.

Of which is also incorrect, E is out.

By POE we arrive at B, which is the correct answer.