The correct option is C Producers
A. Herbivores are primary consumers that feed on producers.
B. Carnivores are secondary consumers that feed on primary consumers.
C. Producers are green plants which synthesise their own food and convert light energy into chemical energy. They are present in the maximum number in an ecosystem because they occupy the lowest trophic level and have the highest amount of biomass or stored energy which is transferred to other organisms from them.
D. Omnivores are the animals that feed on both plants and animals.
So, the correct answer is 'Producers'.