**Core Concept**
Reducing power in pharmacology refers to the ability of a drug to decrease the effect of another drug. This concept is crucial in understanding drug interactions and pharmacodynamics. The reducing power of a drug is often measured by its ability to block or antagonize the action of another drug.

**Why the Correct Answer is Right**
The correct answer is usually a drug that acts as an antagonist to the agonist drug, thereby reducing its effect. For example, if a drug A is an agonist of a receptor, a drug B that acts as an antagonist of that receptor would have reducing power. This is because the antagonist binds to the receptor, blocking the agonist from binding and exerting its effect. The reducing power of a drug is often used in clinical settings to counteract the effects of an overdose or to manage side effects.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not have any reducing power on the agonist drug. Drugs that do not interact with the agonist drug or do not affect its action would not have reducing power.
**Option B:** This option is incorrect because it is actually an agonist itself and would increase the effect of the agonist drug, rather than reducing it.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that reducing power is often a desirable property of a drug in certain clinical situations, such as managing side effects or counteracting an overdose. Antagonists with reducing power can be lifesaving in these situations.

**Correct Answer: C. A drug that acts as an antagonist to the agonist drug.**