A scalar matrix is a square matrix in which all of the principal diagonal elements are equal and the remaining elements are zero.3 x 3 scalar matrix $\begin{array}{ccc} k & 0 & 0 \\ 0 & k & 0 \\ 0 & 0 & k \end{array}$Given $k + 0 + 0 + 0 + k + 0 + 0 + 0 + k = 9$$\Rightarrow 3k = 9$$\Rightarrow k = 3$So, the product of all its elements are $k \times 0 \times 0 \times 0 \times k \times 0 \times 0 \times 0 \times k = 0$