Final answer:
The body's first line of defense against microorganisms is C) skin and mucous membranes, which are part of the innate immune system.
Step-by-step explanation:
The body's first line of defense against microorganisms is C) skin and mucous membranes. These serve as physical and chemical barriers to prevent pathogenic invasion into body tissues. They are part of the innate immune system and work continuously to protect against a broad range of pathogens. If pathogens breach these barriers, the second line of defense, which is an internal defense system, kicks in with a variety of cellular and molecular responses, including inflammation and phagocytosis.