The correct option is C Producers
Generally in the food chain of terrestrial and aquatic ecosystem the largest population is that of producers. The number of organisms in each trophic level of a food chain depend upon the avialability of food and energy. As producers form the base of the energy pyramid, they are generally the largest population. And since energy is lost at each trophic level as heat, the pyramid tapers, indicating a reduction is biomass and hence numbers as you go up the levels.