Answer; The correct answer for the blank is- Food Chain.
Food chain can be described as a series of living organisms that are dependent on one another as a source of food/ obtaining energy for the survival.
In other words, it represents the flow of energy ( transfer of energy in the form of food) among different living organisms present within an ecosystem.
The organisms in the food chain occupy different ranks ( called trophic level) that occur in a particular order ( such as Producers, primary consumers, secondary consumers, tertiary consumers, decomposers).
Thus, food chain is the right answer.