1) Action and reaction forces are a postulate of the third Law of Newton: for every action there is reaction of equal magnitude and opposite direction.
2) (a) a man standing on the ground
It is arbitrary and irrelevant which you considere action and which the reaction.
We will center the analysis on the man, so we will call the force exerted by the man on the ground the action.
The action force, then, is the weight of the man which is exerted on the ground.
So, in virtue of the third Law of Newton, the ground exerts an equal and opposite force on the man. That force is called Normal force.
In conclusion: the action is the weight of the man over the ground; the reaction is the normal force of the ground over the man standing.
3) (b)a stone suspended by a thread from the ceiling.
In this case, centered on the stone, we can call the action the weight of the stone over the thread, and the reaction the tension of the thread.
