**Core Concept**
Conjugation is a phase II reaction in drug metabolism, where a reactive metabolite from a phase I reaction is combined with a molecule such as glucuronic acid or sulfate to enhance water solubility and facilitate excretion. In contrast, phase I reactions involve the modification of the drug molecule through chemical reactions.

**Why the Correct Answer is Right**
Conjugation is a phase II reaction, distinct from the phase I reactions of oxidation, reduction, and hydrolysis. These phase I reactions involve the introduction or removal of a functional group, resulting in a more reactive metabolite, which is then conjugated in phase II. Conjugation reactions, such as glucuronidation and sulfation, occur after the initial phase I modification.

**Why Each Wrong Option is Incorrect**
**Option A:** Hydrolysis is a phase I reaction, where a polar molecule is cleaved to form a less polar molecule, often resulting in the removal of a functional group.
**Option B:** Reduction is a phase I reaction, where an oxidized functional group is reduced, often resulting in the formation of a more stable molecule.
**Option C:** Oxidation is a phase I reaction, where an oxidized functional group is introduced, often resulting in a more reactive molecule.

**Clinical Pearl / High-Yield Fact**
Remember the "two-phase" rule: phase I reactions modify the drug molecule, while phase II reactions conjugate the modified molecule to enhance solubility and facilitate excretion.

**✓ Correct Answer: D. Conjugation**