Which expression can be factorised using the identity a² − b² = (a − b)(a + b)?

Correct answer: y² − 49

The correct answer is A: y² − 49. Rewrite 49 as 7², giving y² − 7². Both terms are perfect squares and they are separated by subtraction, so the difference-of-squares identity applies: y² − 7² = (y − 7)(y + 7). Option B is y² + 7², a sum of squares, not a difference of squares. Option C is actually a perfect square trinomial: y² + 14y + 49 = (y + 7)², so it should use the square-of-a-sum identity. Option D is y² − 14y + 49 = (y − 7)², so it is also a perfect square trinomial, not the difference-of-squares form. The absence of a middle term in A is another useful clue. Memory cue: two squares with a minus sign split into conjugate factors, one with minus and one with plus.