Official Explanation

A very hard question it is, I will rank it a 750+ question, and honestly speaking I haven't seen such a question in any official question bank. Here is the Official explanation.

A very subtle point that will come up on the harder GMAT SC questions is the idea of a summative modifier: a word that “encapsulates” the action of the preceding clause.

To illustrate: the original sentence states that “microbiome…diseases…humanity, amounting…”. In this case, what does “amounting” refer to? Whichever of the three you argue for—assuming you argue for any—none amount to a major oversight in Western medicine. “The microbiome amounts to a major oversight” is odd, though the least odd of the three.

In order to clearly state what is doing the “amounting”, we use a word that “encapsulates” or captures the preceding phrase. This word is known as a summative modifier. In other words, it sums up what is being said. The good news here is you do not need to come up with the summative modifier itself, but you will have to find an answer choice that uses one.

Side note: the GMAT will never have multiple possible summative modifiers, asking you to pick the word that best encapsulates the preceding clause.

In this question, “a discovery” is a perfect summative modifier since it captures the idea of the preceding clause: the micribiome may hold cures to disease, a discovery…”

Just like that we can eliminate (A), (D) and (E).

The difference between (B) and (C) is somewhat similar. But instead of using another summative modifier, which is stylistically off, the correct answer, (C), uses a colon.

As for (B), the “which” refers to the phrase “a major oversight”, since whenever “which” is used, we must make sure that it correctly modifies the subject of the noun phrase that directly precedes it. In this case, the subject of the phrase “a major oversight in Western medicine” is “major oversight”. However, it is not the “major oversight” that is ignoring the role of the microbiome but Western medicine. (C) cleans up this error by clearly stating who has been doing the ignoring: Western medicine.

(B) also uses “a discovery that is amounting to” instead of the more concise “a discovery amounting to” or “a discovery that amounts to”. On the GMAT, the more concise way of phrasing something, as long as there is no change in meaning, will always end up in the correct answer.

Finally, the “until recently + verb tense” is one of those faux errors that test takers can get hung up. There is no hard and fast rule here. Either works.

What the GMAT does want you to know is how to correctly modify clauses. And knowing how a summative modifier works will go a long way.