Which of the following best describes a food chain?

A food chain is best described as a linear sequence showing who eats whom, which is precisely what the correct answer conveys. In a food chain, each organism occupies a specific level, usually referred to as a trophic level, where the flow of energy and nutrients is represented in a direct, step-by-step manner. For example, it begins with a primary producer, such as a plant, which is then consumed by a primary consumer, like an herbivore, and so on through the different levels of consumers. This straightforward depiction allows for clear understanding of how energy is transferred through an ecosystem from one organism to another.

The other choices reference concepts that relate to ecology but do not accurately define a food chain. A complex network of feeding relationships refers to a food web, which encompasses multiple food chains and illustrates the interconnectedness of different species. A cycle of nutrients in an ecosystem pertains to the biogeochemical cycles, which describe how nutrients move through various components of ecosystems but do not specifically illustrate the eating relationships. A classification of living organisms refers to taxonomy, which groups species based on various hierarchical categories rather than showing the direct consumption relationships that a food chain highlights. Thus, the best descriptor of a food chain is the linear sequence showing who eats whom.