Which of the following is used to test chlorine levels in water?

The DPD test, or N,N-diethyl-p-phenylenediamine test, is the appropriate method for testing chlorine levels in water. This test is widely used because it specifically reacts with chlorine, both free and total, to produce a distinct color change that correlates with the concentration of chlorine in the water sample. The intensity of the color formed during the reaction can be quantified either visually or through a colorimeter, thus enabling accurate readings of chlorine levels.

This method is favored in water treatment and quality monitoring because it is sensitive, relatively simple to conduct, and provides rapid results. Other methods, while they may be related to water quality testing, do not specifically target chlorine or do not provide the same level of accuracy for chlorine concentration. Techniques like using a pH meter focus on measuring acidity or alkalinity of the water, while a conductivity meter determines the ionic content and overall salinity levels rather than specific concentrations of chlorine. A colorimeter itself can be useful for various analyses by measuring color intensity, but it is the DPD method that directly measures chlorine levels making it the appropriate answer in this context.