If A is a square matrix such that $A(adj \;A)=\begin {bmatrix} 4 & 0 & 0 \\ 0 & 4 & 0 \\ 0 & 0 & 4 \end {bmatrix}$, then det (adj A) $ is equal to

Question

\[\begin {array} {1 1} (a)\;4 & \quad (b)\;16 \\ (c)\;64 & \quad (d)\;256 \end {array}\]