JAMB 1992 · UME · Q19

Write each term as a product of two consecutive whole numbers: $2 = 1 \times 2$, $6 = 2 \times 3$, $12 = 3 \times 4$, $20 = 4 \times 5$.

The $n$th term is $n(n + 1) = n^2 + n$.

Check $n = 3$: $9 + 3 = 12$.

So the answer is option C.