Torque about an axis

The perpendicular distance from the point of rotation when cross producted with Force given us Torque

So $\tau_{Surface} = \vec{F} \times \vec{r}$ .

Now In these case Angle between $\vec{r}$ and $\vec{F}$ is $90^{o}$

SO $\tau_{CD} = \vec{F} \times 0 = \boxed{0}$

And $\tau_{AB} = \vec{F} \times a = \boxed{Fa}$ where $a$ is the edge of the cube

ABOUT CD ZERO AND ABOUT AB Fa