Solution

Identity matrix

$I_3 = \begin{bmatrix} 1 & 0 & 0 \ 0 & 1 & 0 \ 0 & 0 & 1 \end{bmatrix}$

Determinant of a diagonal matrix = product of diagonal elements

$|I_3| = 1 \times 1 \times 1 = 1$