**Core Concept**
A pro-drug is a pharmacological inactive compound that is converted to its active metabolite in the body. Pro-drugs are designed to improve the bioavailability, stability, or delivery of the active compound. They often require metabolic activation by enzymes in the body to release the active ingredient.

**Why the Correct Answer is Right**
The correct pro-drugs listed among the options are designed to undergo metabolic activation to release their active metabolites. For example, **Option A** is a pro-drug that is converted to its active metabolite by esterases in the body. **Option B** is another pro-drug that is activated by hydrolysis to release its active component. **Option C** is also a pro-drug that undergoes metabolic activation to release its active metabolite.

**Why Each Wrong Option is Incorrect**
**Option D:** This option does not undergo metabolic activation to release an active metabolite. Instead, it is an active compound that does not require conversion to be pharmacologically active.

**Clinical Pearl / High-Yield Fact**
Pro-drugs are often used to improve the pharmacokinetic properties of a compound, such as its bioavailability or stability. They can also be designed to target specific tissues or cell types, allowing for more targeted therapy.

**Correct Answer:** D. This option is not a pro-drug.