**Core Concept**
Cytochrome P-450 is a family of enzymes responsible for the metabolism of xenobiotics, which are foreign substances such as drugs and environmental pollutants. These enzymes catalyze phase I reactions, including oxidation, reduction, and hydrolysis, to activate or inactivate xenobiotics.

**Why the Correct Answer is Right**
The correct answer is A. Cytochrome P-450 because it is the primary enzyme responsible for the oxidation of xenobiotics. This process involves the addition of a hydroxyl (-OH) group to the xenobiotic molecule, making it more water-soluble and thus easier to excrete from the body. Cytochrome P-450 enzymes are embedded in the endoplasmic reticulum of cells in the liver and are dependent on the electron-transporting protein NADPH cytochrome P-450-reductase (option C) to facilitate their catalytic activity.

**Why Each Wrong Option is Incorrect**
**Option B:** Glutathione S-transferase is an enzyme involved in phase II metabolism, where it conjugates glutathione to the xenobiotic molecule, making it even more water-soluble and easier to excrete. However, it is not responsible for the activation of xenobiotics.
**Option C:** NADPH cytochrome P-450-reductase is an electron-transporting protein that is essential for the activity of cytochrome P-450 enzymes, but it is not the enzyme itself that activates xenobiotics.
**Option D:** Glucuronyl transferase is another enzyme involved in phase II metabolism, where it conjugates glucuronic acid to the xenobiotic molecule, making it more water-soluble and easier to excrete. However, it is not responsible for the activation of xenobiotics.

**Clinical Pearl / High-Yield Fact**
The cytochrome P-450 system is inducible, meaning that its activity can be increased by exposure to certain substances, such as barbiturates and phenobarbital. This can lead to increased metabolism of other drugs and potentially reduce their effectiveness.

✓ Correct Answer: A. Cytochrome P-450