Which agent is used to prevent or reduce infection by eliminating or decreasing microorganisms on the skin?

The correct choice is antiseptic because these agents are specifically designed to prevent or reduce infection by either killing or inhibiting the growth of microorganisms on the skin. Antiseptics are typically applied to living tissues and are safe for use on skin, providing a crucial role in preventing infections, especially during procedures that involve breaks in the skin or are conducted in a clinical setting.

In contrast, sanitation generally refers to practices that promote hygiene and minimize the risk of infection, such as cleaning surfaces but does not specifically refer to agents that treat the skin. Disinfection, while effective at killing a broad range of microorganisms, usually pertains to inanimate objects and surfaces rather than living tissue. Wheal refers to a raised, itchy area that can occur during an allergic reaction or skin testing, and is not relevant to the context of infection prevention. Therefore, antiseptics are uniquely suited to the purpose described in the question.