For all integers $ n \leq 1$, which of the following is divisible by 9?

Question

\[\begin {array} {1 1} (a)\;8^n +1 & \quad (b)\;4^n-3n-1 \\ (c)\;3^{2n}+3n+1 & \quad (d)\;10^n+1 \end {array}\]