Explanation
Why A is correct: The author introduces the "equal environments assumption" specifically within the context of the critics' argument. The purpose is to identify a methodological vulnerability in twin studies — not to prove that heritability estimates are wrong (which would be B).
Why B is incorrect: Choice B is a tempting distractor because it concerns heritability estimates. However, the passage does not claim the estimates are inflated — it says the assumption behind them has been challenged. This is a subtler, more limited claim than B suggests.