(b) 3
$a_n = 3n + 7$, so $d = a_{n+1} - a_n = [3(n+1)+7] - [3n+7] = 3$.
The common difference is found by subtracting the $k$th term from the $(k+1)$th term. Since $a_n = 3n+7$ is linear in $n$, the coefficient of $n$ (i.e., 3) directly gives the common difference. The constant 7 is part of the first term, not $d$.