The correct answer is (b). Anaemometer.
A barometer measures atmospheric pressure, a hydrometer measures the density of liquids, and a wind vane measures wind direction.
An anemometer is an instrument used for measuring wind speed and direction. It is typically a device with three or four cups or blades mounted on a rotating frame. The cups or blades are spun by the wind, and the rotation is converted into an electrical signal that is then displayed on a meter or recorded.
There are many different types of anemometers, but they all work on the same basic principle. The cups or blades are spun by the wind, and the rotation is converted into an electrical signal. The electrical signal is then displayed on a meter or recorded.
Anemometers are used in a variety of applications, including meteorology, aviation, and wind energy. They are also used in some industrial applications, such as the measurement of air flow in ventilation systems.