Final answer:
C₃b is the complement component that acts as an opsonin and binds to CR1 on macrophages, thereby marking pathogens for phagocytosis in the innate immune response.
Step-by-step explanation:
The complement component that is an opsonin and binds to complement receptor 1 (CR1) on macrophages is C₃b. The role of C₃b in the complement system is critical for enhancing phagocytosis through opsonization - a process where pathogens are marked to enhance the ability of phagocytic cells such as macrophages to recognize, bind, and ingest them. Complement proteins, including C₃b, serve as markers to indicate the presence of a pathogen, facilitating their phagocytosis by macrophages and other immune cells. This is part of the body's innate immunity that works alongside antibodies to clear pathogens effectively.