B

The torque \( \tau \) acting on a dipole moment \( \vec{p} \) in an electric field \( \vec{E} \) is given by the equation:
Step 1: Understand the formula for torque on a dipole:
\[ \tau = \vec{p} \times \vec{E} \]
Step 2: If the dipole moment is aligned with the electric field (that is, the angle \( \theta = 0 \)), the torque becomes zero since the cross product is zero (\( \sin(0) = 0 \)).
Step 3: However, if the dipole moment makes an angle \( \theta \) with the electric field, the magnitude of the torque is given by:
\[ \tau = pE \sin(\theta) \]
This indicates that the torque depends on both the strength of the electric field and the sine of the angle between the dipole moment and the field direction. Hence, at particular angles the torque can be calculated accordingly.
Conclusion: The torque acting on the dipole moment in an electric field is represented by option B: \( pE \sin(\theta) \).