Adipole of moment 0.5 e·nm is placed in a uniform electric field with a magnitude of 8 times 104 n/c. what is the magnitude of the torque on the dipole when (a) the dipole is parallel to the electric field,

The net torque is zero

Explanation:

Let's assume that the dipole is compose of two equal but oposite charges e, and it cam be represented by a rod with one end having a charge e and the other end with a charge of -e. Notice that the dipole is parallel to the electric field thus the force felt by both of the charges will be parallel to the electric field. This means that there will be no components of the forces that are perpendicular to the rod which is a requirement for it to have a torque.