Concept:A footbath at a farm entrance is a biosecurity measure used to kill germs carried on footwear.Explanation:A footbath contains a chemical solution that destroys disease-causing microorganisms on boots.Such chemicals are known as disinfectants.Herbicides kill weeds, antiseptics are safe for living tissue, and antibiotics treat internal infections.Thus, the correct substance in a farm footbath is a disinfectant.Answer:D. disinfectants