The diagonal (in $\text{cm}$) of a cuboid with dimensions $9\text{cm}\times 12\text{cm}\times 20\text{cm}$ is:

The diagonal $d$ of a cuboid with length $l$, width $w$, and height $h$ is given by the formula:
$$d=\sqrt{l^2+w^2+h^2}$$
Given dimensions: $l=9$, $w=12$, $h=20$.
$$d=\sqrt{9^2+12^2+20^2}$$
$$d=\sqrt{81+144+400}$$
$$d=\sqrt{625}$$
$$d=25\text{cm}$$