This is an arithmetic sequence with common difference 2. The nth term is first term + (n − 1) × difference = 9 + (12 − 1) × 2 = 31. An arithmetic sequence changes by the same amount each step. Use aₙ = a₁ + (n − 1)d, where a₁ is the first term, d is the common difference, and n is the term number. Do not multiply the common difference by n directly; there are only n − 1 jumps from the first term to the nth term. The correct answer is 31, which matches the calculation or reasoning shown.