What type of larviciding product forms a coating on the water's surface to suffocate mosquito larvae and pupae?

The correct answer is oils, as they are specifically designed to form a thin film on the surface of the water. This film disrupts the breathing process of mosquito larvae and pupae, effectively suffocating them. Oils create a barrier that prevents these aquatic stages of mosquitoes from accessing the air they need, leading to their elimination.

Granules, emulsions, and suspensions generally do not operate by forming a surface coating. Granules are solid particles that sink to the bottom and may not have the same suffocating effect on larvae situated at the surface. Emulsions are mixtures that contain tiny droplets of oil dispersed in water, but they do not form a continuous film on the water's surface like oils do. Suspensions consist of solid particles dispersed in a liquid, which also do not coat the surface in the same effective manner as oils, thus lacking the mechanism needed to suffocate mosquito larvae and pupae.