Besides being an excellent source of calcium and protein, yogurt, when

Official Explanation:

Read the original sentence carefully, looking for errors:

The introductory phrase "Besides being an excellent source of calcium and protein" logically refers to yogurt, so "yogurt" has to follow immediately after the comma.

Scan and group the answer choices:

If it weren't clear that (A) is the only choice that properly places "yogurt" after the introductory modifier, there is also a 3-1-1 split at the end of the underlined portion (the singular "helps replenish," the plural "help replenish," and the wordy "helps in replenishing").

Eliminate answer choices until only one remains:

(B) states that "a person" is an excellent source of calcium and protein. This may be true, but it is not the intended meaning of the sentence. Also, this adds a recommendation, with "should," that the author never explicitly makes in the original.

(C) uses the phrase "helps in replenishing," which is unnecessarily wordy. Also, this choice incorrectly changes the meaning of the sentence to imply that yogurt is an excellent source of calcium and protein only when it's consumed during or after an illness.

(D) introduces an ambiguous pronoun "it," which could refer to either the yogurt or an illness, and it has the same problem as (C), suggesting that yogurt is only helpful in conjunction with illness.

(E) incorrectly changes the source of nutrients to "consuming yogurt."

(A) is the correct answer.

TAKEAWAY: A modifying phrase at the beginning of a sentence should always be followed immediately by the thing to which it refers.