<p>Pronoun reference rules are not hard and fast, but here are some guidelines. In general, we want the referent of a pronoun to be as close to the pronoun as possible. This proximity preference finds exception, though, when the referent appears in a different clause than the pronoun. Such is the case here, when we prefer the referent of a pronoun that serves as the subject of the independent clause (“it was once used…” for this question) to be the subject of its dependent clause. </p>
<p>For example, when one says, “Bob gave Limbo his shoes yesterday, but he is usually rather stingy,” it is assumed in the absence of contravening context that the subject pronoun “he” of the independent clause refers not to the nearer “Limbo” of the dependent clause but instead the subject of that clause, “Bob.”</p>
<p>Choice (A) is undesirable because the subject pronoun “it” of the independent clause refers not to the subject of the dependent clause (“damage”) but to the object of a prepositional phrase, “strip-mining.” Other problems include the unnecessary use of the passive voice (“caused by…”), which is not ungrammatical but is wordy, and the non-standard phrase “irreparable to.” </p>
<p>Choice (B) has the same pronoun reference ambiguity and passive-voice verbosity. It also has non-standard phrasing: “caused…to” (notwithstanding the colloquial practice of saying that damage was “caused to” something).</p>
<p>Choice (C), while not completing removing all doubt as to the referent of “it,” observes better pronoun-reference practices by making the subject of the dependent clause (“strip-mining”) the referent of the subject of the independent clause. It also removes the problematic “caused to” and “irreparable to” in favor of the more agreeable “damage to.” Finally, it’s in the active voice.</p>
<p>Choice (D) does not explicitly indicate that strip-mining is the cause of the damage, which is integral to the intended meaning of the sentence. Also of issue is that “fact of” is non-standard phrasing.</p>
<p>Choice (E) exhibits the same lack of necessary causal conveyance as choice (D). Inserting “causing” before “irreparable” to create a participial connection would probably create the ideal choice. Nonetheless, choice (C) is the clear favorite among our offerings.</p>
<p>Hope this helps.</p>