b
Dipole moment of the dipole \(=\vec p\) and uniform electric field \(=\overrightarrow E\).

We know that dipole moment \((p) = q.a\) (where \(q\) is the charge and \(a\) is dipole length).

And when a dipole of dipole moment \(\overrightarrow p \) is placed in uniform electric field \(=\overrightarrow E\) ,

then Torque \((\tau ) =\) Either force \(\times \) perpendicular distance between the two forces

\(= qaE\,sin\,\theta \) 

\(\tau \, = \,pE\,\sin \,\theta \)

\(\vec \tau \, = \,\vec p \times \vec E\) (vector form)