thangvietnam It's true that sometimes we have to compare the answer choices to see whether the GMAT offers a clearer option. For pronouns, that generally means that if we can replace a pronoun with the noun it refers to, we should probably do so. That option generally means that the GMAT considers the pronoun in question unclear or incorrect.
However, we don't have to pay any special attention to what A says. Answer choice A is no more or less likely to be correct than any other, and there is no reason to treat it as "the original." Just pick the choice that works!
Also, while we can see this error by comparison, we can also say that in general, the GMAT doesn't want us to use the same pronoun (for instance, variations of "they") to refer to two different things. We do this all the time in real life ("She told her that she was ready.") and as long as the meaning is clear, it's not really a problem, but on the GMAT, this is typically a violation.