**Core Concept**
An agonist is a type of drug that binds to a receptor and activates it, resulting in a biological response. This is in contrast to an antagonist, which blocks the receptor and prevents activation. Agonists can be full agonists, which produce a maximal response, or partial agonists, which produce a submaximal response.

**Why the Correct Answer is Right**
The correct answer is related to the concept that agonists produce a biological response by activating the receptor. The mechanism of agonist action involves binding to the receptor and inducing a conformational change that activates the receptor and triggers a signaling cascade. This can lead to various physiological effects, depending on the type of receptor and the agonist involved. In the context of neurotransmission, agonists can activate postsynaptic receptors to produce a response in the target cell.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the effect of an agonist. Agonists do not just "block" the receptor, but rather activate it.
**Option B:** This option is incorrect because it is too vague and does not specifically relate to the effect of an agonist. While agonists can produce a biological response, this option does not provide any specific information about the mechanism of action.
**Option C:** This option is incorrect because it is not a characteristic of an agonist. Antagonists, not agonists, are typically used to block the receptor and prevent activation.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that agonists can have varying degrees of efficacy, ranging from full agonists to partial agonists, which can have different effects on the body.

**Correct Answer:** D.