Although often miles beneath the ocean's surface, a seasoned marine

Blair15
That isn't a rule--pronouns are different from modifiers. If the subject were plural (archaeologists), we would assume that "they" referred to the subject, so it would be confusing to use "they" to refer to another plural noun that hadn't shown up yet. Since the subject is singular, we know to wait to find the correct plural antecedent for "they." Also, since we are only able to adjust the initial portion of the sentence, we really only have two choices: rewrite the modifier to refer to an archaeologist (doesn't make sense here) or turn that portion into a clause with its own subject. Only C does that, so we have to cut all the others.

Having said that, this sentence does seem deliberately confusing. As an editor, I would flag this for rewrite. I think that if the GMAT presented this initial sentence, the correct answer would be reordered for greater clarity, but that would require a longer underline.