The correct answer is: A. Settleable solids.
An Imhoff cone is a device used to measure the amount of settleable solids in a liquid. It is a cone-shaped container with a scale on the side. The liquid is poured into the cone and allowed to settle for a specified time. The amount of solids that settle to the bottom of the cone is then measured.
Settleable solids are solids that can be removed from a liquid by sedimentation. They are typically composed of particles of clay, silt, and organic matter. Settleable solids can cause problems in wastewater treatment plants, as they can interfere with the treatment process and clog filters.
Suspended solids are solids that are not removed by sedimentation. They are typically composed of particles of sand, silt, and organic matter. Suspended solids can cause problems in wastewater treatment plants, as they can interfere with the treatment process and make the effluent cloudy.
Dissolved solids are solids that are dissolved in a liquid. They are typically composed of ions of salts, acids, and bases. Dissolved solids can cause problems in wastewater treatment plants, as they can interfere with the treatment process and make the effluent salty.