What is a "stimulus"?

The term "stimulus" refers to any external factor or condition that elicits a response from an organism or system. This could be a physical sensation, like a change in temperature or light, or it could involve more abstract triggers, such as emotional or cognitive stimuli. The fundamental aspect of a stimulus is its ability to provoke an action or reaction, making option C the most accurate definition.

In biological contexts, stimuli can include anything from environmental changes to internal signals that prompt responses such as movement, reflex actions, or emotional reactions. The presence of a stimulus is integral to the process of interaction between an organism and its environment, highlighting its role as a catalyst for behavior or reaction.