Researchers have questioned the use of costly and experimental diagnos

I'm just going to reproduce this with the underlining in place:
Researchers have questioned the use of costly and experimental diagnostic tests to identify food allergies, such as milk, that supposedly disrupt normal behavior.
(A) to identify food allergies, such as
(B) to identify food allergies, like
(C) to identify food allergies, such as to
(D) for identifying food allergies, like that of
(E) for identifying food allergies, such as for

The MGMAT approach usually starts with splits, but sometimes we make exceptions. In particular, when the original sentence includes a parallel marker, we know that the stuff after that marker must be structurally similar to some earlier logically comparable stuff. In this case, the phrase such as is a sort of parallel marker. Whatever follows the such as has to be an example of food allergies.
A) Milk is not an allergy. Eliminate A.
B) This still offers milk as an example of an allergy, or perhaps compares allergies to milk. Either way, it's wrong. Eliminate B.
C) As awkward as this sounds, it's actually defensible. Such as to here means such as (an allergy) to. Don't eliminate C.
D) That stands in for allergies, creating two problems. First, that is singular while allergies is plural. Second, allergy of is unidiomatic. Eliminate D.
E) Allergy for is also unidiomatic. Eliminate E.

C it is.

By the way, I imagine that some began by eliminating B and D, because they use like rather than such as to introduce an example. That's actually a complicated issue.
First, every time that I have seen a choice between like and such as on a real GMAT SC, the right answer used such as.
BUT second, in at least one OG explanation, GMAC acknowledges that like can introduce examples in some circumstances.
BUT third, you should not use like to introduce examples unless you mean those examples to be restrictive. If I say I enjoy movies like Miller's Crossing, I mean I enjoy movies relevantly similar to Miller's Crossing. I'm not just giving you an example of a movie, but an example of the sort of movie I enjoy.
SO finally, like is wrong for the sentence above. Feel free to eliminate B and D for that reason.

Oh, and a small point about daagh's answer. I agree with him that this seems an unlikely question, but the comma before that isn't a problem, because the comma doesn't introduce the that clause, it closes the phrase right before the that clause.

Want more detail? Daagh is correct that you shouldn't use a comma to introduce a restrictive clause, and that relative clauses that begin with that are restrictive, but the comma isn't used to introduce the clause here. Rather, the pair of commas are used to set off the phrase such as to milk. Without that phrase, the correct answer would read, Researchers have questioned the use of costly and experimental diagnostic tests to identify food allergies that supposedly disrupt normal behavior. No comma problems.