In the United States one of the earliest challengers of the economic

https://www.nytimes.com/1987/07/22/business/economic-scene-trade-theory-from-harvard.html

FREE trade has been a venerated economic principle in the United States going back 200 years. But there have been challengers. The first was Alexander Hamilton, who in 1791 argued for government policies that would encourage native manufacturing while protecting it from British exports.

Verb form; Agreement; Parallelism

The point of the sentence is that Alexander Hamilton challenged free trade by advocating government policies that would encourage indigenous manufacturing and, at the same time, protect this manufacturing from competition from British exports. Some of the answer choices use the plural pronoun them, but the antecedent of the pronoun is manufacturing, which is singular, so the singular pronoun it should be used. As given, the sentence correctly uses the singular pronoun it, but, because the sentence's predicate is a compound predicate, in which two verb phrases are conjoined by the word and, the same verb form should be used, which is not the case here.

(A) Because the predicate is compound, with verb phrases conjoined by and, the verb forms should be the same; thus, to protect is incorrect. The pronoun it is correct, however, as its antecedent is manufacturing.

(B) Because the predicate is compound, with verb phrases conjoined by but, the verb forms should be the same; thus, to protect is incorrect. The pronoun it is correct, however, as its antecedent is manufacturing.

(C) Correct. The sentence is well expressed. The pronoun it is correct as its antecedent is manufacturing, and the use of the participle phrase while protecting it from competition from British imports (the subject, indigenous manufacturing, is given earlier and need not be repeated) appropriately conveys the idea that Hamilton advocated policies that protected the manufacturing at the same time as they encouraged its development.

(D) The use of them is incorrect. Since it is plural the antecedent cannot be the singular indigenous manufacturing, though that would appear to be the intended antecedent (the idea underlying the sentence is that Hamilton, in a challenge to the principle of free trade, sought both to encourage and protect indigenous manufacturing). To take either of the plural nouns government policies or earliest challengers as the antecedent renders the sentence illogical, as does the use of the preposition for. Although it might make sense to think that encouraging indigenous manufacturing would protect the United States, United States, like manufacturing, is singular, and so cannot serve as the antecedent of them.

(E) As with answer choice D, there is no plural noun that is a logically plausible antecedent for the plural pronoun them.