How will the right answer fit in terms of support and conclusion?

Only the right answer will be a conclusion supported by the passage.

Highlight the main conclusion in the passage, if there is one:

There is no conclusion in the passage. The right answer will be the conclusion.

[BACKGROUND]. [BACKGROUND]. [SUPPORT]. [SUPPORT].

Map the wording of the answers to the wording of the passage:

(A) Boiled cow’s milk makes less vitamin C available…

This doesn’t bring in anything new or make any extreme statements. And this conclusion would be reasonable if the switch to boiled cow’s milk was followed by a “vitamin C deficiency”.

(B) …frequently contract potentially fatal…

The passage only says that preventing these infections was the idea behind boiling the cow’s milk. But there’s no info on how often these infections happen, so no way you can make a conclusion that says it happens “frequently”.

(C) Mother’s milk can cause gastrointestinal infections in infants.

The only info given about “mother’s milk” is that infants eating it “did not contract scurvy”. There’s no support for saying it causes infections.

(D) …they did not know that scurvy was caused by…

There’s no information about what doctors did or “did not know”.

(E) …most mothers did not breast-feed their infants.

There are absolutely no numbers mentioned or referenced in the passage, so there’s definitely no support for saying “most mothers” did anything.

(A) is the correct answer.