Assertion (A): A man standing on a completely frictionless surface can propel himself by whistling. Reason (R): If no external force acts on a system, its momentum cannot change.

Assertion A is true because when a man whistles, he expels air from his mouth. According to Newtons Third Law of Motion, for every action, there is an equal and opposite reaction. The air pushed out provides a backward force on the man, allowing him to move even on a frictionless surface. Reason R is true as it states the Law of Conservation of Momentum. This law explains that the total momentum of a system remains constant if no external force acts on it. Reason R is the correct explanation for Assertion A because the system consists of the man and the air inside him. When the air is expelled, it gains momentum in one direction. To keep the total momentum of the system constant at zero, the man must gain momentum in the opposite direction. Therefore, the internal force of whistling results in movement due to the conservation of momentum.