In physics, the phenomenon of a gun recoiling backward when fired is a classic example of Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction. Let's explore this principle and its application in this scenario to understand why the backward motion occurs:

1. Understanding the Action: When a bullet is fired from a gun, the bullet is propelled forward with a certain force. This force is generated by the expanding gases from the gunpowder exploding in the cartridge, pushing the bullet through the barrel.
2. Newton's Third Law of Motion: According to this law, if object A exerts a force on object B, object B simultaneously exerts a force of equal magnitude and in the opposite direction on object A. In this case, the bullet (object B) exerts a forward force on the gun (object A).
3. Reaction Force and Recoil: The reaction force exerted by the bullet on the gun is what causes the gun to move backward. This backward movement of the gun is known as recoil.
4. Ruling Out Other Options:
   • Friction between gun and person: While friction affects how the gun is handled, it does not directly cause the backward motion.
   • Air resistance: This affects the bullet's trajectory but not the initial recoil of the gun.
   • Gravity acting on the gun: Gravity is constant and does not change due to firing the bullet, so it does not cause recoil.
5. Conclusion: The recoil of the gun is primarily due to the reaction force of the bullet on the gun, complying with Newton's Third Law of Motion.

Thus, the correct answer for the backward motion of the gun when fired is due to the reaction force of the bullet on the gun.