The correct answer is D) Anemometer. An anemometer is the instrument used to measure wind velocity, which is a critical piece of information for Coast Guard captains when navigating vessels. Anemometers measure the speed of the wind by using cups or propeller blades that spin in response to the moving air, and the rotation speed is then converted to a wind speed measurement. The other answer choices are incorrect because: A) A wind sock indicates wind direction but not velocity, B) a barometer measures atmospheric pressure, and C) a psychrometer measures humidity, not wind speed.