The correct answer is C. An anemometer
Explanation
An anemometer is a device that determines the speed of the wind, anemometers are usually composed by a set of revolving cups or vanes that twirl according to the wind and that activate an electric generator or a counter that determines the speed of the wind. On the other hand, weather vanes and windsocks are instruments use to know the direction of the wind, but no its speed and barometers are used to determine the atmospheric pressure, which is not related to wind.
