A change or signal in the environment that causes a reaction.

• A. Response
• B. Digestion
• C. Nutrients
• D. Stimulus

Answer: (D)

A stimulus is any change or signal in the environment that triggers a reaction from a living organism. It can be something you can sense directly, like a bright light, a loud sound, a touch, or a chemical cue, and organisms respond to these cues to survive or adapt. When you see a bright light, your pupils constrict; when you touch something hot, you pull your hand away. These responses begin because receptors detect the stimulus and send signals that lead to a behavior or physiological change.

The other terms are not the trigger itself. A response is the organism’s action after sensing a stimulus. Digestion is a metabolic process for breaking down food. Nutrients are substances organisms use for energy, growth, and repair. So the best choice for “a change or signal in the environment that causes a reaction” is the stimulus.