HI vipulgoyal,

This SC is based on modification and "style" rules. Since the entire sentence is underlined, we have the "opportunity" to add/subtract words, reorder the words and even change the meaning of the sentence. In this situation, we're looking for an answer that is grammatically correct from beginning to end.

In these situations, it's sometimes difficult to "group and eliminate" answer choices, so you have to go on the hunt for individual errors in each sentence.

A: "Using a metal detector" is a modifying phrase that describes "hobbyists." Here though, it modifies "old coins." Old coins cannot use a metal detector. Eliminate A.

B: "Old coins...can be located BY hobbyists" is written in passive voice, which is rarely correct. In this sentence, the wording should be "hobbyists can locate old coins...." Eliminate B.

C: "Using a metal detector" correctly describes "hobbyists." The rest of the sentence IS in active voice. The pronoun "they" may seem like a vague pronoun, but "they" refers to the preceding plural noun "coins and other valuables" and any perceived ambiguity here is silly ("hobbyists" are not buried in the sand and dirt.

D: "....can be located BY hobbyists" is passive. Eliminate D.

E: The sentence doesn't state WHO is using a metal detector. With no subject, we have our first problem. The phrase "...BY a hobbyist" is also passive. Eliminate E.

Final Answer: C

GMAT assassins aren't born, they're made,
Rich