Which of the following is an agent used to extinguish Delta fires?

Delta fires, which involve flammable liquids such as fuels, oils, and greases, require specific firefighting agents for effective extinguishment. The use of sand as an extinguishing agent is primarily due to its ability to suffocate the fire by cutting off the oxygen supply. When sand is poured onto a Delta fire, it creates a barrier that prevents the flammable vapors from escaping into the atmosphere, effectively smothering the flames.

Other agents like water can exacerbate Delta fires due to the potential for oil to float on water, spreading the fire further. AFFF (Aqueous Film-Forming Foam) is designed for use on class B fires, which include flammable liquid fires, but the question pertains to a general extinguishing method. CO2 is effective for certain types of fires, particularly electrical and some flammable gas fires, but it is not the preferred option for Delta fires. Therefore, using sand aligns with established safety practices for these types of fires, making it the most suitable answer in this context.