Let $M$ and $N$ be two $3\times 3$ non singular skew-symmetric matrices such that $MN=NM$.If PT denotes the transpose of P,then $M^2N^2(M^TN)^{-1}(MN^{-1})^T$ is equal to

Question

$(a)\;M^2\qquad(b)\;-N^2\qquad(c)\;-M^2\qquad(d)\;MN$