When you sit on a chair what is the reaction force to your weight (the downward action force)?

Question

The frictional force between the chair and the floor.
The tension force in your muscles.
The force of gravity.
The upward force exerted by the chair on your body

Answer 1

The reaction force to your weight (the downward action force) when you sit on a chair is the upward force exerted by the chair on your body.

According to Newton's third law of motion, for every action, there is an equal and opposite reaction. Your weight acts downward due to gravity, and in response, the chair exerts an upward force on you, which supports your weight.