The secchi disc is a simple, circular disk (usually with alternating black and white quadrants) that is lowered into the water to measure its transparency, also known as water clarity. The depth at which the disk is no longer visible from the surface is called the Secchi depth, and this measurement is an indicator of the amount of suspended material (turbidity) in the water.

A. A hygrometer is an instrument used to measure the amount of humidity or moisture content in the air. It is not used for measuring water transparency.

B. A barometer is a tool for measuring atmospheric pressure. It has no application in measuring water transparency.

D. An anemometer is an instrument used to measure wind speed and direction. It is unrelated to water transparency.