What is the factorised form of m² − 9?

Correct answer: (m − 3)(m + 3)

The direct answer is D: (m − 3)(m + 3). First recognise that 9 = 3², so m² − 9 becomes m² − 3². This is a difference of two squares. Apply A² − B² = (A − B)(A + B), with A = m and B = 3: m² − 3² = (m − 3)(m + 3). Expanding option D gives m² + 3m − 3m − 9 = m² − 9, so it is verified. Option A gives m² − 9m, not the original expression. Option B gives m² − 8m − 9 and introduces an unwanted middle term. Option C gives (m − 3)² = m² − 6m + 9, which is a square of a difference, not a difference of squares. Memory cue: a² − b² always produces conjugate factors with opposite signs.