Submarine exploration reveals both that hot water vents through the se

In option D, The "it" refers to Hot water vents right? Isn't that plural here ?

"It" refers to "hot water" (singular). "Vents" is a verb here. Notice that if it were a noun (vents of hot water), there would be no verb and therefore we wouldn't have a complete sentence.

D for me.Both X and Y is the correct idiom and also parallelism of that clause is maintained in this choice.

D has - both ... and....
idiom both x & y always confuses me.... isn't that redundant?

Well... When "both" refers to two verbs, using "and" is actually the only way you can apply the form "both this and that" correctly.
In fact, "both" is the extra flavor here, being there simply to strengthen the connection between the verbs... the important part, grammatically speaking, would be the "and".
You're probably thinking about a phrase in terms like "both boys dated the same gal", perhaps. Do note that "both" here is an adjective referring to a noun, not to two verbs.

For further references: https://dictionary.cambridge.org/it/gram ... nnico/both

Hope it helped.

Submarine exploration reveals both that hot water vents through the sea floor at great depths, but also that it supports diverse life forms in the complete absence of light.

There are two idioms:
1) Both ..........and.......
2) Not only ...........but also..........

This sentence has combined two different idioms.

(A) both that hot water vents through the sea floor at great depths, but also that it supports diverse life forms
(B) both hot water venting through the sea floor at great depths, as well as their supporting diverse life forms
(C) that the support of diverse life forms at great depths where hot water vents through the sea floor
(D) both that hot water vents through the sea floor at great depths, and that it supports diverse life forms
(E) that hot water both vents through the sea floor at great depths, and the support of diverse life forms

I have understood why choice d is a correct one, but when you notice choice “c”, the word both is missing and as per the question, it is required. Could this also be the reason that choice c is incorrect? I feel that choice “c” is grammatically correct. Correct me if I am wrong..

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KAPLAN OFFICIAL EXPLANATION:

D

In the original sentence, we see the phrase “both that,” so we should immediately check to make sure that it’s followed by “and that.”It is not in (A) or (B), but it is in (D). Choices (C) and (E) omit the word “both,” which is okay, but these choices have other problems. Even without the “both . . .and” formula, the things listed have to be grammatically parallel. Since (E) begins with “that,” it should repeat the word “that”for the second piece of information and have “supports” to parallel “vents,” but it doesn’t. As for (C), it leaves the “that” clause with no main verb.

Though "both " sounds a bit confusing here, we may solve this question by framing the sentence in a simple parallelism structure as follows:

S exploration reveals BOTH THAT X does so .... AND THAT Y does so....
If we go by this rule we see "D" as the answer as no other option follows this structure. The point to be noted here is "IT" refers to the "hot water" (IT -- singular. Submarine Exploration and Hot Water are singular, but Submarine exploration can't support a life form. Hence IT refers to the HOT WATER here. Also check the placeholder rule). Another point to be noted here is that "Vents" is a verb in this sentence.

In my opinion, the following option may also be correct:
S exploration reveals THAT X does so .... AND Y does so.... ... Since there is no such option mentioned on this parallelism structure, hence D