Answer: The correct answer for the fill in the blank is Producers.
Producers form the first trophic level in a food chain. They are capable of synthesizing food through the process of photosynthesis and thus called autotrophic organisms. These organisms capture energy from sunlight and convert it into chemical energy in the form of ATP and NADPH that is utilized in the formation of food, which is a carbohydrate.
This food is utilized by next trophic level and the flow of energy takes place in this manner.
So by synthesizing food they form an integral part of the food chain as they feed all other life forms.
Thus, in a food chain, flow of energy begins with producers, which feed all life.
Examples of producer- Green plants, algae, cynobacteria.