I'd say it's an error in the mark scheme, I do OCR A but we are taught that the third law is that "If object A were to exert a force on object B, then object B would exert an equal but opposite force on object A", with the conditions being that the forces are of the same type (contact, gravitational), and act on two different bodies. So a pair would be the force of earths gravity on a ball, and the force of the balls gravity on earth.
I'd say the answer is B.