The correct answer is: D. 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 base area of 100 square centimeters and a height of 20 centimeters. The cone is filled with the liquid to be tested, and the time it takes for the solids to settle to the bottom of the cone is measured. The amount of settleable solids is then calculated by multiplying the volume of the settled solids by the density of the liquid.
Total solids are the total amount of matter in a liquid, including both dissolved and suspended solids. Total organic solids are the total amount of organic matter in a liquid. Total inorganic solids are the total amount of inorganic matter in a liquid.
Settleable solids are the solids in a liquid that will settle to the bottom of the container under the influence of gravity. 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.