Concept:Insecticide particles spread through the air in the cupboard from a region of higher concentration to lower concentration.Explanation:When the insecticide is sprayed, its particles mix with air and move randomly.This movement carries the particles from the sprayed area to all parts of the cupboard.Since the cockroaches are at a distance from the spray source, the particles reach them by spreading through the air.This net movement of particles from a high-concentration region to a low-concentration region is called diffusion.Answer:C. diffusion