Newton's Third Law

Newton's Third Law states that for every action there is an equal and opposite reaction, and crucially these two forces act on two different bodies. When the swimmer pushes a mass of water backward, that water simultaneously pushes the swimmer forward with an equal magnitude force, propelling the swimmer ahead. The option invoking gravity is wrong because gravity acts vertically and plays no role in horizontal propulsion. The claim that water exerts no force is incorrect, since the reaction of the displaced water is precisely what drives forward motion. The option saying the two forces cancel misunderstands the law: action and reaction never cancel because they are applied to different objects, here the water and the swimmer separately. This mirrors the NCERT explanation of swimming, walking, and rocket propulsion as Third Law phenomena. As a consistency check, the more vigorously the swimmer pushes water back, the stronger the forward reaction, which agrees with the everyday observation that harder strokes produce faster swimming.