The given situation involves two action-reaction pairs of forces. They are :(a) The action force (F = mg) with which the boy pushes the chair in the downward direction and the reaction force (F' =−mg) with which the chair exerts on the boy in the upward direction.(b) The action force (F = (M + m)g) with which the boy and the chair push the ground in the downward direction and the reaction force (F' = (M + m)g) with which the ground exerts on the boy and the chair in the upward direction.