If $A=\left[\begin{array}{ccc}2& 0& 0\\ 0& 2& 0\\ 0& 0& 2\end{array}\right]$ and $B=\left[\begin{array}{ccc}1& 2& 3\\ 0& 1& 3\\ 0& 0& 2\end{array}\right]$, then $|AB|$ is equal to:

Using the property of determinants $|AB|=|A||B|$.$|A|$ is the determinant of a diagonal matrix, which is the product of its diagonal elements:
$$|A|=2\times 2\times 2=8$$
$|B|$ is the determinant of an upper triangular matrix, which is the product of its diagonal elements:
$$|B|=1\times 1\times 2=2$$
Therefore:
$$|AB|=|A|\times |B|=8\times 2=16$$