The correct option is C Rainforests
Primary producers are the number of autotrophs at first trophic level in an ecosystem which produce or synthesise organic matter. In a specified area of a desert ecosystem the number of producers are very less when compared to the grassland ecosystem where mostly the producers are the grasses thus the herbivores are more dependent on the grasses. However the diversity of producers is low in a grassland, compared to a forest ecosystem. In comparison to all the above mentioned ecosystems the number of producers are highest in rainforests that show high plant diversity and hence greater number of producers relatively.