To accurately determine what the piece of equipment is used for, I would need to know the specific type of equipment you are referring to. Each option corresponds to a different meteorological instrument:
A. A thermometer measures air temperature. B. A hygrometer measures the amount of moisture in the air. C. An anemometer measures wind speed. D. A rain gauge measures the amount of rain that falls over a period of time.
If you can specify which equipment you are asking about, I can provide a more precise answer.