I think option B is the correct choice.

The main point of concern is the starting comparison of the upstrokes in hummingbirds - " A does more of X than B does" this is the correct form best used to compare A & B (hummingbirds vs other birds) - The only change here is that sicne both A & B are plural, it changes to

" A do more of X than B do" which can be re-structured into "A do more of X than do B" . Only option B has this correct usage; so the others are ruled out. Option A is incorrect because of singular verbs (allows, gives..) being used for plural noun (hummibgbirds)

Hope it helps!
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Though B is the best among the choices mentioned, I don't think this is a good question.
"comma + Which" modifies the noun before it and is incorrectly used in most of the choices. Here which seem to modify the complete clause.

In case anyone was curious, here's the Official Explanation for the question.

Answer: B - The comparison in the first part of the sentence is between the lift that hummingbirds produce and the lift that other birds produce. Thus, the second half of the comparison requires not only "other birds," but also a verb, such as "do" in the sentence as written. That eliminates choices (C) and (E), both of which compare the lift that hummingbirds produce directly to "other birds," not the lift that other birds produce. Choice (D) also represents an incorrect comparison. "Instead of" means that one thing is done in place of another. It isn't accurate to say that hummingbirds produce more lift in place of producing the lift of other birds. There are two differences between (A) and (B). First, the construction of the second clause is more efficient in (B), as it places "hover" and "extract" as parallel to one another. Second, "whether they be" in (A) contains an ambiguous pronoun, since "they" could refer to "hummingbirds." Choice (B) is correct.

Also as per MGMAT use of command subjunctive after whether is old fashioned, chapter 7 page 114.
Whether it be chocolate, vanilla or any other flavor. << wrong
Whether it is chocolate, vanilla or any other flavor. << right

I eliminated option A as per above mentioned rule.

Option B is correct.

I have a doubt here : which refers to preceding noun always, but here "which means" is referring to whole idea in preceding clause; is it fine ?

'More than' is the right idiom. C D and E are out.

In option A, 'which' referee to 'birds' and that is not right.

Hence option B is correct
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Why is comma + which correct here? It should refer to the nearest noun but it is referring to an action. I found this incorrect. Pls explain.

Posted from my mobile device

I agree with you. Present participle (verb-ing) is required here. This question is not GMAT-like and does not seem to be from an authentic source.

Hi all,

in Option B, "which" relative pronoun modifies which noun?

Please clarify

B. than do other birds, which means that hummingbirds have the capability to hover and extract nectar from plants, whether

I chose option B at first but I was confused about this part of the sentence.

',whether honeysuckle,sage, or fuchsia. '

is this a complete sentence?

I thought it should have been - where it is honeysuckle,sage, or fuchsia

D and E can be easily discarded because of their uses of ‘instead’. ‘Instead’ implies that the hummingbirds are replacing the other birds in some way.
C compares the lift produced to the other birds, so it is out too. A has an ambiguity problem. It’s not clear if ‘they’ is referring to the birds or the flowers.

B is the right answer.

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