**Core Concept**
An agonist is a type of ligand that binds to a receptor and activates it, leading to a biological response. This is in contrast to antagonists, which block the receptor and prevent the response.

**Why the Correct Answer is Right**
The correct answer is an agonist because it binds to the receptor and causes a conformational change that activates the receptor. This activation triggers a signal transduction pathway that ultimately leads to a biological response. The agonist's shape is complementary to the receptor's binding site, allowing it to bind and activate the receptor.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because antagonists do not activate the receptor, but rather block it, preventing the biological response.
* **Option B:** This option is incorrect because inverse agonists are a type of ligand that binds to the receptor and reduces the receptor's activity below the baseline level, even in the absence of an agonist.
* **Option C:** This option is incorrect because partial agonists are a type of ligand that binds to the receptor and activates it, but to a lesser extent than a full agonist.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the potency of an agonist is related to its ability to bind to the receptor and cause a conformational change that activates the receptor.

**Correct Answer: C. Partial agonist**