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 $\left[\begin{array}{ccc}\mathit{k}& 0& 0\\ 0& \mathit{k}& 0\\ 0& 0& \mathit{k}\end{array}\right]$
Given $\mathit{k}+0+0+0+\mathit{k}+0+0+0+\mathit{k}=9$
$\Rightarrow 3\mathit{k}=9$
$\Rightarrow \mathit{k}=3$
So, the product of all its elements are $\mathit{k}\times 0\times 0\times 0\times \mathit{k}\times 0\times 0\times 0\times \mathit{k}=0$