The potential at point a is φA=-kQR+kQd? R=-kQ(d？ 2R)R(d？ r)； The potential at point C is φC=-kQR+kQd+R=-kQdR(d+R).

Then the potential difference between a and c is UAC=φA-φC=-kQ(d? 2R)R(d？ R)-(-kQdR(d+R))=2kQRd2？ R2，

Protons move from a to c, and the electric field force does work as WAC=eUAC=2kQeRd2? R2 is positive work, so the proton potential energy decreases by 2kQeRd2? R2, so A is correct.

So choose: a.