Schistosomiasis, a disease caused by a parasitic worm, is prevalent in

I don’t have a whole lot to say about (A). The first thing I notice is the modifier “that are the parasite’s hosts” – and that seems like a reasonable way to modify “the freshwater snails.”

The pronoun “its” also jumps out at me, and that would have to refer to “the parasite’s”, since that’s our nearest (and most logical) singular. And that’s fine, too: “…the freshwater snails that are the parasite’s hosts for part of [the parasite’s] life cycle…” Reasonable enough.

So I guess we’ll keep (A).

The biggest difference between (B) and (A) is the plural pronoun “their” in (B). And that’s a problem: “their” is plural, so it has to refer to either “the freshwater snails” or “the parasite’s hosts” – and those are the same thing, anyway. So that gives us: “… the freshwater snails that are the parasite’s hosts in part of [the freshwater snails’] life cycle.”

That’s not WRONG, exactly, if you’re willing to assume that the sentence is trying to explain what happens during the snails’ life cycle. But I think that misses the point: schistosomiasis is the grammatical subject of the sentence, and the sentence is clearly trying to explain why the disease has become more common. So presumably, the sentence is interested in explaining that the snails are hosts during part of the parasite’s life cycle. The snail’s life cycle really isn’t of interest to us, and wouldn’t help explain why the parasite has become more common.

So that’s miserably subtle, but it’s basically all we’ve got, unless you want to make an unnecessarily big deal out of the prepositions. (A) is better than (B), so we can eliminate (B).

(C) is a little bit easier to eliminate than some of the other options. The phrase “its life cycles” really doesn’t make any sense at all: “its” presumably refers to “the parasite”, since that’s the closest (and most logical) singular noun. But then how is that “the parasite” has multiple “life cycles”? As far as I know, each parasite only has one life cycle. If not, that would be creepy AF.

There’s also a problem with the phrase “freshwater snails which becomes.” Snails are plural, so the verb would have to be “become”, not “becomes.” Plus, “which” is generally a non-restrictive modifier, so it usually follows a comma – though that’s not usually a deciding factor on GMAT questions.

So we have lots of reasons to ditch (C).

(D) is a little bit better than (C)… but only a little bit.

I guess it’s nice that “its life cycles” has been changed to “the parasite’s life cycles”, but either way, it’s illogical: a singular parasite presumably has only one life cycle, not multiple “life cycles.”

It’s also nice that the subject-verb issue has been fixed (“snails… become”), but I still don’t think it’s ideal to use “which” in this type of scenario without a comma. But again: the GMAT doesn’t generally make a big deal out of comma placement, and it’s almost never a deciding factor.

And even if you ignore the comma thing, the phrase “parasite’s life cycles” is goofy enough to let us eliminate (D).

The plural pronoun “their” appears twice in (E), but “parasite’s” is actually singular, so “their” presumably refers to “snails.” And that’s garbage: “… freshwater snails which become [the snails’] hosts during [the snails’] life cycles…”

You could also argue that “which” needs to be preceded by a comma, as we mentioned in (C) and (D) – but again, that’s rarely a deciding factor on the GMAT.

The pronoun thing is a big deal, though. (E) is out, and we’re left with (A).