A stimulus is

A stimulus is

• A. An external factor that causes a reaction
• B. An internal factor that causes a reaction
• C. Any external or internal factor that causes a living organism to react
• D. A factor that causes a reaction

Answer: (C)

A stimulus is any detectable change that leads to a response in a living organism. It can come from outside the body (like light, temperature, touch, or a chemical in the environment) or from inside the body (such as changes in hormone levels, blood sugar, or CO2 concentration). The key idea is that the organism senses something and that detection prompts a reaction. That’s why the best answer includes both external and internal factors that can trigger responses. Saying it’s only external or only internal misses part of how organisms respond, and calling it merely “a factor that causes a reaction” is too vague because it doesn’t specify that the factor is something the organism perceives as a signal.

A stimulus is any detectable change that leads to a response in a living organism. It can come from outside the body (like light, temperature, touch, or a chemical in the environment) or from inside the body (such as changes in hormone levels, blood sugar, or CO2 concentration). The key idea is that the organism senses something and that detection prompts a reaction. That’s why the best answer includes both external and internal factors that can trigger responses. Saying it’s only external or only internal misses part of how organisms respond, and calling it merely “a factor that causes a reaction” is too vague because it doesn’t specify that the factor is something the organism perceives as a signal.