All, I tried the question below and opted for option B. The logic I used was that the sentence before the comma has a singular subject while the second part is talking of islands. Hence used plural verb

The OA is A

4. Although the vast constellation of small islands that dot the South Pacific has a population of only a few million people, it is home to about a sixth of the world’s 6,000 or so languages
a. Same
b. Pacific has a population of only a few million people, they are
c. Pacific is populated by a only a few million people, and it is
d. Pacific, having a population of only a few million people, is nonetheless
e. Pacific, populated by only a few million people, yet they are

Please underline your question. It will be much appreciated if you provided the answer at the bottom after stating the question and answer choices instead of at the very top.

4. Although the vast constellation of small islands that dot the South Pacific has a population of only a few million people, it is home to about a sixth of the world’s 6,000 or so languages
a. Same
b. Pacific has a population of only a few million people, they are
c. Pacific is populated by a only a few million people, and it is
d. Pacific, having a population of only a few million people, is nonetheless
e. Pacific, populated by only a few million people, yet they are

Thanks for posting it neatly. Makes it much better to read and as we all are trying to learn and test ourselves , always better to post OA after question & the answer choices. So agree with "agha79" on those points.

Now in my view the question is testing two things one as you pointed out S-V agreement. And the main subject is the "The constellation ".
Also i have noticed that GMAT uses Although for 2 main purposes , one to introduce a subordinate clause so as a subordinate conjuction and second for contrast.

A. Although.....(sub clause) , it....(Main clause). Also the contrast is very clear.
B. They doesnt agree with main subject
C. The comman & AND is wrong, we need two independent clauses if we want to use a coordinate conjuction
D. Awkard, for me if you remove the -ing modifier, its a fragement (other people might have better explanation for this choice so lets see if we get more inputs)
E. Although and Yet both show contrast so redundant and They doesnt agree with main subject.

Thanks ankurbansal84 for updating the question.
B & E are out because of Subject verb agreement. For “C” as Nitan described I don’t see any other issue other than coordinate conjunction.

(b) has a weird subject shift - the subject suddenly changes from "constellation" to "islands" - but it doesn't really seem "wrong enough" to be a wrong answer on a real question.

Hi Ron,

This is a GMAT Prep question. I got it on Practice test 2. To be honest, I don't see a problem with "they are" in B. Can you explain any other reason to eliminate B please?

Thanks,
Vikas

B is wrong as "they are" assumes the subject is the people. But the constellation is the subject as it is "the home to ..." and not the people.

yeah, but:

* it doesn't say "THE home to...", it just says "home to..."
(this actually doesn't make any difference here; i'm just pointing it out)

* it's perfectly possible for several islands (or several of something else) to be "home to" something. i.e., if you have "X is/are home to...", then there is no guarantee that X is singular.
for instance:
these row houses, which were once home to the city's elite, are now home to a fearsome street gang.

so - as i stated above, the only thing wrong with this aspect of (b) is the weird subject shift that takes place.
i.e.
you can totally say that the constellation of islands is home to X people, but you can also say that the islands (themselves) are home to X people.
the former is only better in that "the constellation..." is already the subject of the preceding clause, so that the sentence "flows" better.

--

on the other hand, the pronoun in (a) ("it") is also better than the pronoun in (b) ("they").

in (a), "it" is technically ambiguous between "vast constellation" and "South Pacific", but there is GRAMMATICAL PARALLELISM between "it" and "vast constellation" because each of these is the subject of its respective clause. (click here for more details on this "acceptable pronoun ambiguity".)

in (b), "they" is also technically ambiguous, between "small islands" and "a few million people". however, in this instance, neither of these words is grammatically parallel to the pronoun: the pronoun is the subject of its clause, but neither of these possible antecedents is the subject of its own clause. so this is not as good.