Question

A and B are two bodies .A exerts a force on B by F due north. What is the Force on A by B.

Answer 1

Thanks for asking the question.
The force exerted by body B on body A  will be same as the force exerted by body A on body B that is F but opposite in direction that is SOUTH. This can be explained by Newton's third law of motion that is action and reaction are equal and opposite. So the force exerted by B is actually the reaction force.
Hope it helps.