A square matrix $B=[bij]_{n×n}$ where $bij =0 $ when i#j $bij= k$ when $i=j$ for some constant k is called :

Diagonal matrix Identity matrix Scalar matrix Null matrix

Diagonal matrix

The correct answer is Option (1) → Diagonal matrix $B_{n×n}=\begin{bmatrix}k&0&0...&0\\0&k&0...&0\\....\\0&0&0...&k\end{bmatrix}$ so $B=KI_n$ so B is a diagonal matrix