**Core Concept**
Inverse agonists are a class of drugs that bind to the same receptor as an agonist but induce a pharmacological response opposite to that of the agonist. This occurs because inverse agonists stabilize a receptor conformation that is distinct from the active state induced by agonists.

**Why the Correct Answer is Right**
Inverse agonists decrease the basal activity of a receptor, which means they reduce the receptor's activity in the absence of an agonist. This is in contrast to neutral antagonists, which simply block the action of an agonist without affecting the receptor's basal activity. Inverse agonists work by binding to the receptor and inducing a conformational change that reduces the receptor's ability to respond to an agonist. This is achieved by stabilizing a receptor conformation that is distinct from the active state induced by agonists.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because inverse agonists do not simply block the action of an agonist; they actually decrease the receptor's basal activity.
**Option B:** This option is incorrect because inverse agonists do not activate the receptor; they decrease its activity.
**Option C:** This option is incorrect because inverse agonists do not have a zero effect on the receptor; they actually decrease its activity.

**Clinical Pearl / High-Yield Fact**
Inverse agonists should be used with caution, as they can decrease the receptor's activity even in the absence of an agonist, which can lead to decreased efficacy of the receptor.

**Correct Answer:** C. Inverse agonists decrease the basal activity of a receptor.