Answer
$A=\left[\begin{array}{lllll}
a_{11} & 0 & 0 & 0 & 0\\
0 & a_{22} & 0 & 0 & 0\\
0 & 0 & a_{33} & 0 & 0\\
0 & 0 & 0 & a_{44} & 0\\
0 & 0 & 0 & 0 & a_{55}
\end{array}\right]$
Work Step by Step
The elements $A_{ii}$ in a square n$\times$n matrix form the main diagonal.
$A_{ij}$, when $i\neq j$, are the entries not on the main diagonal.
Here,
$A=\left[\begin{array}{lllll}
a_{11} & 0 & 0 & 0 & 0\\
0 & a_{22} & 0 & 0 & 0\\
0 & 0 & a_{33} & 0 & 0\\
0 & 0 & 0 & a_{44} & 0\\
0 & 0 & 0 & 0 & a_{55}
\end{array}\right]$
