b
The torque of electric force about centre is balanced by torque due to friction about the centre

\(r f ={ PE } \sin \theta\)

\(\tau= PE \sin \theta\)

\(\text { But } f = mg \sin \theta\)

\(mgr \sin \theta= PE \sin \theta\)

\(E =\frac{ mgr }{ P }=\frac{ mgr }{ q \times 2 r }\)

\(P = q \times 2 r (\text { dipole moment })\)

\(E =\frac{ mg }{2 q }\)