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: A man standing on a completely frictionless surface can propel himself by whistling. When the man whistles, he expels air from his lungs. This expulsion of air creates momentum in one direction. By the law of conservation of momentum, the man must acquire an equal and opposite momentum to keep the total momentum of the system (man + expelled air) constant (assuming it was initially zero). This opposite momentum causes the man to move in the direction opposite to the expelled air. Reason (R) is true: If no external force acts on a system, its momentum cannot change. This is a statement of the principle of conservation of momentum.