(c)

Work done in rotating a dipole , =pE(1-cosq)

If q = 90°, work done = pE (1- 0) = pE

Dear Student,
Please find below the solution to your problem.

Workdone = ΔpE
pE = −p​E
pEf​ = 0
pEi​ = −pE
ΔpE = pE

Thanks and Regards