The matrix $P=\begin{bmatrix}0 & 0 & 4\\0 & 4 & 0\\4 & 0 & 0\end{bmatrix}$ is a

Question

$$\begin{array}{1 1}(A)\quad square\;matrix & (B)\quad diagonal\;matrix\\(C)\quad unit\;matrix & (D)\quad none\end{array}.$$