The correct answer is power because the sound of the lifeguard's whistle has the ability to command attention and compel action. In a pool or beach setting, the lifeguard's whistle is a symbol of authority and control. When it cuts through the air, it signifies that someone in a position of power is issuing a directive or warning. The use of the word "shrill" suggests a sharp, piercing sound, emphasizing the strength and commanding nature of the whistle's power.