Which of the following categories does the recursive algorithm we used to solve the Tower of Hanoi problem belong to with respect to the number of discs $n?$

$($a$)O(n)$

$($b$)O(n^2)$

$($c$)O(2^n)$

$($d$)O(n!)$

$($a$)$

$($b$)$

$($c$)$

$($d$)$