I think this is a question of what's "less wrong." The point here is that we need to use present perfect ("have done") to show that it started in the past and still continues today. Everything else is secondary.

I agree that the pronoun "they" is ambiguous. But, it's the "most correct" (or the "least wrong") of all the answer choices.

The ansswer choices are clearly breaking cardinal rules and I dont understand. I guess since all of them are guilty of same sin, we need to pick least horrible answer choice. But do I have the right idea as to my objection to the answer choice?

I think A is correct choice,even they is referring to closest noun i.e. wine growers.

Remember the sentence correction hierarchy:

(1) grammar
(2) meaning
(3) style

Pronoun ambiguity is a style issue, not a grammar issue (i.e. it doesn't violate grammar rules, it just makes the sentence a bit tougher to understand). Therefore, the correct answer to a SC question may not be as stylistically perfect as it could be - but it will always be stylistically superior to the other grammatically correct/true to original meaning answers.

The GMAT is known to tolerate pronoun ambiguity when the meaning of the sentence is clear. A person who's fluent in english recognizes that grapes can't perform actions, but wine growers can, so the meaning of the sentence is clear.

The GMAT will not tolerate pronoun ambiguity in cases in which the sentence could have multiple meanings. For example:

"Bob hates Fred becase he is an idiot"

is ambiguous, because it's possible that Bob is an idiot or that Fred is an idiot.