Concept:Turbidity is the cloudiness or haziness of water caused by suspended particles.
It is measured by checking how much light can pass through the water.
Explanation:A Secchi disc is a circular plate with black and white quadrants.
It is lowered into water until it is no longer visible.
The depth at which it disappears shows the level of transparency.
Lower visibility depth means higher turbidity.
Therefore, the Secchi disc is used to determine water turbidity and transparency.
The other instruments serve different purposes.
A hygrometer measures humidity in air.
A hydrometer measures the density or specific gravity of liquids.
A rain gauge measures the amount of rainfall.
Answer:Secchi disc
(C)