Answer
$\tau = 0$
Work Step by Step
$p$ is the dipole moment
$E$ is the electric field
Note that when the dipole moment is anti-parallel to the electric field, the angle $\theta$ between the directions is $\theta = 180^{\circ}$
We can find the torque on the dipole when the dipole moment is anti-parallel to the electric field:
$\tau = p \times E$
$\tau = p~E~sin~\theta$
$\tau = p~E~sin~180^{\circ}$
$\tau = 0$
