Final answer:
Examples of biological barriers in the first line of defense include skin, mucus, and stomach acid, which prevent pathogen invasion.
Step-by-step explanation:
The question is asking for examples of biological barriers involved in the body's first line of defense against pathogens. The correct answer is A) Skin, mucus, and stomach acid. These are part of the body's innate immune system and serve as initial physical and chemical barriers to prevent pathogenic invasion into the body tissues. Skin acts as a mechanical barrier, mucus traps microbes and facilitates their removal, and stomach acid provides a harsh environment that kills many pathogens.