What is defined as any detectable change in an organism resulting from the activities of a pathogen or pest?

The correct choice is defined as a "symptom," which refers specifically to the observable changes or effects in an organism that result from the presence or activity of a pathogen or pest. Symptoms can manifest in various forms, including physical changes like lesions, wilting, or discoloration, and are essential for diagnosing the health of a plant or the impact of pest infestations.

Understanding symptoms is crucial in the context of pest management and plant health assessments, as they guide the identification of the specific pest or pathogen causing the issue and inform management decisions. For example, recognizing that yellowing leaves may be a symptom of a nutrient deficiency or a pest-related issue is fundamental for effective treatment strategies.

The other options, while related to biological responses, do not specifically denote the visible changes that occur in an organism due to pests or pathogens. The term "effect" often conveys a broader cause-and-effect relationship rather than the specific manifestations observed. "Response" typically refers to a general action or change that might not be observable, while "reaction" can imply a more immediate action taken in response to a stimulus rather than the extended changes associated with disease or pest presence.