Questions: A specific condition in the internal environment to which a receptor responds is called a(n) stimulus

The answer is the third one (or C): stimulus.

Explanation for each option:

• Antagonist: This term typically refers to a substance that interferes with or inhibits the physiological action of another, especially in the context of neurotransmitters or hormones. It is not the correct term for a condition in the internal environment to which a receptor responds.
• Precursor: This term refers to a substance from which another is formed, especially by metabolic reaction. It does not describe a condition that a receptor responds to.
• Stimulus: This is the correct term. A stimulus is any change in the internal or external environment that can be detected by receptors and elicits a response.
• Catalyst: This term refers to a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change. It is not the correct term for a condition that a receptor responds to.

Summary: A specific condition in the internal environment to which a receptor responds is called a stimulus.