If $A$ and $B$ are square matrices of the same order and $AB=3I$ then $A^{-1}$ is equal to

Question

$(a)\;3B\qquad(b)\;\large\frac{1}{3}$$B\qquad(c)\;3B^{-1}\qquad(d)\;\large\frac{1}{3}$$B^{-1}$