$\{ n(n+1)(2n+1): n \in Z \} \in$

Question

$$\begin {array} {1 1} (a)\;\{6k:k \in Z\} & \quad (b)\;\{12k:k \in Z\} \\ (c)\;\{18k:k \in Z\}& \quad (d)\;\{24 k:k \in Z\} \end {array}$$