**Core Concept**
Selenium is an essential trace element that acts as a cofactor for certain enzymes, particularly those involved in antioxidant defense and redox reactions. These enzymes are critical for maintaining cellular homeostasis and protecting against oxidative stress.

**Why the Correct Answer is Right**
One of the key enzymes that rely on selenium as a cofactor is glutathione peroxidase (GPx). GPx plays a crucial role in reducing lipid hydroperoxides and hydrogen peroxide, thereby preventing cellular damage caused by reactive oxygen species (ROS). Selenium is incorporated into the active site of GPx as selenocysteine, a unique amino acid that is essential for the enzyme's activity. This selenocysteine residue is responsible for the catalytic activity of GPx, enabling it to reduce peroxides and maintain cellular redox balance.

**Why Each Wrong Option is Incorrect**
* **Option A:** This enzyme is not known to require selenium as a cofactor. While it may be involved in similar redox reactions, it does not rely on selenium for its activity.
* **Option B:** This enzyme is involved in a different biochemical pathway and does not require selenium as a cofactor. Its activity is not dependent on the presence of selenium.
* **Option C:** This enzyme is involved in a different type of redox reaction and does not require selenium as a cofactor. Its activity is not dependent on the presence of selenium.

**Clinical Pearl / High-Yield Fact**
Selenium deficiency can lead to impaired glutathione peroxidase activity, resulting in increased oxidative stress and cellular damage. This can manifest as various clinical conditions, including muscle weakness, fatigue, and increased risk of cardiovascular disease.

**Correct Answer: C. Glutathione peroxidase.**