**Core Concept**
Thioredoxin reductase is a crucial enzyme in the thioredoxin system, which plays a vital role in maintaining cellular redox balance. This enzyme catalyzes the reduction of oxidized thioredoxin, thereby facilitating the repair of damaged proteins and protecting against oxidative stress.

**Why the Correct Answer is Right**
The correct answer is Selenium (Se), an essential trace element that is a critical component of the thioredoxin reductase enzyme. Selenium is incorporated into the enzyme's active site, where it serves as a cofactor for the reduction of thioredoxin. The presence of selenium in thioredoxin reductase is essential for its catalytic activity and plays a key role in maintaining cellular antioxidant defenses.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as there is no established link between thioredoxin reductase activation and the amino acid Lysine.
* **Option B:** This option is incorrect as there is no established link between thioredoxin reductase activation and the amino acid Arginine.
* **Option C:** This option is incorrect as there is no established link between thioredoxin reductase activation and the amino acid Tryptophan.

**Clinical Pearl / High-Yield Fact**
Selenium deficiency is associated with increased oxidative stress and impaired thioredoxin reductase activity, leading to various clinical manifestations, including muscle weakness, cardiomyopathy, and immunodeficiency.

**Correct Answer:** D. Selenium.