C

To find the torque () acting on a dipole moment  in an electric field , we can use the formula:
Torque (\tau) = \vec{p} \times \vec{E}
where \( \tau \) is the torque, \( \vec{p} \) is the dipole moment, and \( \vec{E} \) is the electric field.
When the dipole moment is aligned with the electric field, the angle between them is zero. Thus, the sine of the angle becomes zero as well. This results in zero torque:
Torque = pE \sin(0) = 0
Thus, the correct answer is zero.
Therefore, option C is correct.