If I3 denotes identity matrix of order 3 × 3,

In an identity matrix, all the diagonal elements are 1 and rest of the elements are 0.
Here,

$I_{3}=\mid \begin{array}{lll}1 & 0 & 0\end{array}$

$\begin{array}{lll}0 & 1 & 0\end{array}$

$\begin{array}{lll}0 & 0 & 1\end{array}$

$=1 \times \mid \begin{array}{ll}1 & 0\end{array}$

$\begin{array}{ll}0 & 1\end{array} \quad\left[\right.$ Expanding along $\left.\mathrm{C}_{1}\right]$

$=1$

$\Rightarrow \mathrm{I}_{3}=1$