Which of the following is a key role of the complement system?

The complement system plays a critical role in the immune response by enhancing the ability of antibodies and phagocytic cells to clear pathogens from an organism. This process, known as opsonization, involves complement proteins binding to pathogens' surfaces, which marks them for easier recognition and ingestion by phagocytes such as macrophages and neutrophils. This enhancement of phagocytosis is crucial for effectively eliminating infections and is a definitive aspect of how the immune system protects the body.

In contrast, infant nutrition is primarily addressed through maternal milk, while stimulating B cell proliferation relates to the adaptive immune response and is mainly driven by signals from other immune cells or antibodies rather than complements. Transporting oxygen is a function of the hemoglobin in red blood cells, unrelated to the immune functions that the complement system serves. Hence, the key role of the complement system in enhancing phagocytosis is essential for an efficient immune response.