Final answer:
Option (3), The skin uses several chemical factors to resist infection, including an acidic pH and lysozyme in secretions, but complement proteins are not a chemical factor of the skin for resisting infections.
Step-by-step explanation:
The skin serves as a significant barrier to infection, providing both physical and chemical defenses against pathogens. Among the chemical factors contributing to the skin's resistant properties are the acidic pH of the skin's surface, which inhibits bacterial growth, the enzyme lysozyme found in sweat, saliva, and tears that breaks down bacterial cell walls, and the sebum produced by sebaceous glands that contains antimicrobial properties.
However, while sebum does have some antimicrobial components, it also contains compounds that certain microbes can use as nutrients. Among the options provided, complement proteins are not a chemical factor that helps the skin to be resistant to infections; instead, they are part of the immune system's response once pathogens have already entered the body.