What is the primary role of plants in an ecosystem?

The primary role of plants in an ecosystem is to generate oxygen through photosynthesis. During this process, plants convert carbon dioxide and sunlight into glucose and oxygen. This oxygen is essential for the survival of most living organisms, as it is used in cellular respiration, the process by which animals and humans convert food into energy while releasing carbon dioxide as a byproduct.

While plants do contribute to nutrient cycles and can play a role in cleaning the air by absorbing pollutants, their most fundamental function is as primary producers in the food chain, which directly involves the generation of oxygen. The production of waste products is generally not a primary role of plants, as they primarily serve to sustain life rather than act as a source of waste. Thus, option D accurately captures the core ecological importance of plants in sustaining life within an ecosystem.