The correct answer is A) Force. According to the kinetic molecular theory, pressure is the total force exerted by particles in constant motion upon the wall of their container, divided by the area of that wall.
According to the kinetic molecular theory, the pressure on a wall of a container occupied by particles in constant motion is the force on the wall divided by the area of the wall. Therefore, the correct answer to the student's question is A) Force.
The kinetic theory of gases explains that gas particles are always in motion. When these particles collide with the walls of their container, they exert a force. The total force applied by all particles, when divided by the area of the wall, results in what we understand as pressure. As such, pressure is fundamentally a measure of force per unit area.