What does conductivity measure in a substance?

The measurement of conductivity in a substance specifically refers to its ability to conduct electric current. In the context of water treatment and other related fields, conductivity is primarily associated with the presence of ions in a solution, which facilitate the flow of electricity. This property is critical in assessing water quality, as higher conductivity often indicates a greater concentration of dissolved salts, minerals, or impurities.

While the options listed include various forms of conductivity, the one relating to the ability to conduct electric current is the most relevant. For example, the ability to conduct heat, light, or sound refers to different physical properties altogether and does not apply to what conductivity measures in the context of solutions or treatments. Understanding this concept is vital for analyzing water treatment processes, evaluating treatment efficiency, and ensuring compliance with safety standards.