The correct answer is A) Water. Water is the most effective extinguishing agent for a mattress fire because it effectively cools the burning materials and prevents the fire from spreading. Mattress fires typically involve solid combustible materials that can be extinguished by applying water, which absorbs the heat and prevents reignition. The other options are less effective for this type of fire. Dry chemical extinguishers are more suitable for flammable liquid or electrical fires, foam is better for hydrocarbon-based fires, and CO2 extinguishers are primarily used for electrical or small, clean-up fires. Water provides the best cooling and coverage to fully extinguish a mattress fire.