Parts per million (ppm) by mass means the number of milligrams of solute per kilogram of solution (or equivalently, grams per million grams). So 1 ppm = 1 mg/kg for aqueous solutions where the solvent density approximates water. Given a limit of 1.5 ppm, the maximum fluoride allowed per kilogram of water is 1.5 mg/kg. For 2.0 kg of drinking water: maximum fluoride = 1.5 mg/kg × 2.0 kg = 3.0 mg. Option 1.5 mg corresponds to the fluoride in only 1.0 kg of water, not 2.0 kg. Option 0.75 mg results from a factor-of-two error in the opposite direction (perhaps dividing instead of multiplying). Option 30.0 mg would correspond to 15 ppm for 2 kg, ten times the safe limit. The concept of ppm as a concentration unit appears in NCERT Chapter 2 as part of the various ways to express concentration of solutions, and it is environmentally significant — ppm is widely used for trace contaminants, pollutants, and water quality. Plausibility check: 3.0 mg in 2.0 kg gives 3.0/2000 g/g × 10⁶ = 1.5 ppm, correctly matching the given limit.