So first of all, a quick way to tell A is wrong is because it mentions "earlier north american agricultural techniques" when the question stimulus mentions nothing about earlier agricultural techniques, but rather earlier north american bison numbers. You would have to think that by the author eluding to bison meat yielding the same amount as cattle, that is with current techniques.

Second, E is right not because it says it could be LARGELY reversed, but because the logic that the author used is there. The author's main point was that we destroyed the ecosystem to make room for cattle, but if we started that reversal back, we wouldn't suffer in meat production, because cattle and bison produce the same amount of meat.

So if you really think about it, E could have also been right if it said the ecosystem could be BARELY reversed, or ENTIRELY reversed, the logic is the same so it doesn't matter.

Also, why are you thinking about pesticides?? the question never mentions pesticides, machinery, or earlier agricultural methods, so they definitely won't be in the answer.

When looking at LR, you should imagine that you are a complete blank minded alien, (that knows logic of course) that only knows what the author is telling you, bring no outside information or assumptions in.