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# $8^n-7n-1$ is divisible by ? where $n\in N$

$\begin{array}{1 1} 8 \\ 49 \\ 343 \\ none\;of\;these \end{array}$

Can you answer this question?

$8^n-7n-1=(1+7)^n-7n-1$
$=(1+7n+^nC_2.7^2+.......^nC_n.7^n)-7n-1$
$=^nC_2.7^2+^nC_3.7^3+.......^nC_n.7^n$
which is divisible by $7^2=49$