The correct answer is C. Potentiometer.
A potentiometer is an instrument used to measure the potential difference (voltage) between two points in an electric circuit. It can be used to measure the pH of a solution by comparing the potential difference between a reference electrode and a pH electrode. The pH electrode is a glass electrode that is sensitive to the hydrogen ion concentration in the solution. The reference electrode is a constant potential electrode that does not change with the pH of the solution. The potentiometer measures the potential difference between the two electrodes and converts it to a pH reading.
An Imhoff cone is a device used to measure the settleability of sewage. It is a cone-shaped container with a graduated scale on the inside. The sewage is poured into the cone and the time it takes for the solids to settle is measured. The settleability of sewage is an important indicator of the quality of the sewage.
A turbid meter is a device used to measure the turbidity of a solution. Turbidity is a measure of the amount of suspended particles in a solution. The turbidity of sewage is an important indicator of the quality of the sewage.
Therefore, the correct answer is C. Potentiometer.