Ecosystem

A food chain is series of plants/animals which are interrelated in the form of organism being eaten as food by the other. Plants are producers. They producer their own food. They are present in the first trophic level. The animals eating plants are called as primary consumers and animals eating primary consumers are called as secondary consumers. Decomposers are those which decomposes the waster organic matter. In a food chain, the largest population is that of producers. Hence, usually the pyramid is upright. 

Thus, the correct answer is 'Producers.'