Which of the following acts as a protective barrier against the entrance of microorganisms?

The protective barrier against the entrance of microorganisms is primarily provided by the skin. The skin serves as the body's first line of defense, acting as a physical barrier that prevents pathogens from entering the body. It is composed of multiple layers that are tough and impermeable, which deters the penetration of various microorganisms, including bacteria and viruses.

Additionally, the skin plays an important role in immune response, producing antimicrobial substances and facilitating healing through various mechanisms. Thus, its function in protecting against infection and maintaining overall health is critical and multifaceted.

Mucous membranes, while also protective, serve specific roles in areas such as the respiratory, gastrointestinal, and genitourinary tracts, where they can trap pathogens and particles but are generally not as robust as the skin in providing a complete barrier. Hydrochloric acid is important in the digestive system for breaking down food and killing microorganisms in the stomach but does not act as a barrier in the same way the skin does. White blood cells are essential components of the immune system that respond to infections rather than acting as barriers.