The correct answer is (d) either a magnetic or an electrostatic force.
Explanation:
Electrostatic force and magnetic force can both cause repulsion between objects. Electrostatic repulsion occurs between objects with the same electric charge. Magnetic repulsion occurs between magnets with the same pole facing each other.
Why other options are incorrect:
(a) frictional force only: Friction is a force that opposes relative motion between two surfaces. It always acts in the direction opposite to the motion, meaning it can never cause repulsion.