Answer
$\tau = p~E$
Work Step by Step
We can find the torque about the center due to the electric field $E$ acting on the $+q$ charge:
$\tau = (q)~(E)~(\frac{s}{2})$
$\tau = \frac{q~s~E}{2}$
By the right hand rule, this torque is into the page.
We can find the torque about the center due to the electric field $E$ acting on the $-q$ charge:
$\tau = (q)~(E)~(\frac{s}{2})$
$\tau = \frac{q~s~E}{2}$
By the right hand rule, this torque is into the page.
We can find the magnitude of the net torque:
$\tau = \frac{q~s~E}{2}+\frac{q~s~E}{2}$
$\tau = qs ~E$
$\tau = p~E$
