**Core Concept**
Xenobiotics are foreign substances that can be harmful to the body. To eliminate these substances, the liver uses a metabolic pathway called biotransformation to convert them into more water-soluble compounds that can be easily excreted.

**Why the Correct Answer is Right**
The liver uses two main phases in the biotransformation of xenobiotics. In Phase I, enzymes such as cytochrome P450 (CYP450) catalyze reactions that introduce or expose a functional group (-OH, -NH2, -SH) to the xenobiotic, making it more reactive. In Phase II, conjugation reactions occur, where the xenobiotic is linked to a molecule such as glucuronic acid, sulfate, or glycine, making it even more water-soluble. This process is crucial for the elimination of xenobiotics from the body.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the primary goal of xenobiotic metabolism. While some xenobiotics may be excreted in their original form, the primary purpose of biotransformation is to make them more water-soluble for excretion.

**Option B:** This option is incorrect because it suggests a single-step process, whereas biotransformation involves multiple enzymatic reactions in both Phase I and Phase II.

**Option C:** This option is incorrect because it implies that xenobiotics are converted directly into a waste product without the involvement of enzymes or conjugation reactions.

**Clinical Pearl / High-Yield Fact**
A key aspect of xenobiotic metabolism is the concept of enzyme induction and inhibition. Certain substances can induce the production of CYP450 enzymes, increasing the metabolism of xenobiotics, while others can inhibit these enzymes, leading to decreased metabolism and potentially serious side effects.

**Correct Answer:** D. Conjugation.