For a matrix $M=[m_{ij}]; \: i,j=1,2,3,4$, the diagonal elements are all zero and $m_{ij}=-m_{ji}$. The minimum number of elements required to fully specify the matrix is ________

For a matrix $M=[m_{ij}];i,j=1,2,3,4$, the diagonal elements are all zero and $m_{ij}=−m_{ji}$ (This is the property of skew-symmetric matrix). Now, $M= \begin{bmatrix} 0 & -m_{21} & -m_{31} & -m_{41} \\ m_{21} & 0 & -m_{32} & -m_{42} \\ m_{31} & m_{32} & 0 & -m_{43} \\ m_{41}& m_{42} & m_{43} & 0 \end{bmatrix}$ Out of $16$ elements, $4$ are $0s$ and $6$ are the negative of the other $6.$ So, we need at least $6$ elements to fully specify the matrix. So, the correct answer is $(B).$