Final answer:
The normal flora serves as a physical and chemical barrier in the immune system's first line of defense, occupying space and utilizing resources to prevent the colonization of pathogenic microbes.
Step-by-step explanation:
The first line of defense in the immune system consists of physical and chemical barriers that are always at work to protect the body against a broad range of pathogens. Among the options provided, normal flora (B. Normal Flora) serves as both a physical and chemical barrier for immunity and is part of the first line of defense.
These biological barriers include harmless bacteria that occupy spaces and use resources that could otherwise be used by pathogenic microbes, preventing them from colonizing the body. This forms part of the innate immune response discussed under barrier defenses, which are continuously active and do not adapt to specific pathogens like the adaptive immune system does.