The population within this ecosystem is the three snails. Hence the correct option is d.
Within the depicted ecosystem, the population is represented by the three snails. A population, in ecological terms, refers to a group of individuals of the same species living in a specific area. In this case, the three snails form a distinct population within the aquatic ecosystem. These snails play a role in the community by contributing to the cycling of materials and participating in the balance of the ecosystem.
While the ecosystem includes various components such as guppies, catfish, aquatic plants, and an air stone, the focus on the snails as a population emphasizes their collective presence and interactions within the environment. The snails contribute to nutrient cycling and may impact the health of aquatic plants by grazing on algae, demonstrating their ecological significance in maintaining a balanced and functioning ecosystem. Hence the correct option is d.