Correct Answer: Residual chlorine

Solution : The correct option is residual chlorine.

The quantity of chlorine that remains in water after disinfection is known as "residual chlorine." The concentration of chlorine (typically in the form of hypochlorous acid and hypochlorite ions) that remains in the water after disinfection is referred to as residual chlorine. Parts per million (ppm) or milligrammes per litre (mg/L) are often used to quantify residual chlorine levels.