**Core Concept**
Cystine is a type of amino acid that is formed through the oxidation of two cysteine molecules. This process involves the loss of electrons, resulting in the formation of a disulfide bond between the two cysteine residues.

**Why the Correct Answer is Right**
The correct answer involves the oxidation of cysteine to form cystine. This reaction is facilitated by enzymes such as cystine oxidase, which catalyzes the removal of electrons from cysteine, resulting in the formation of cystine. The disulfide bond between the two cysteine residues is a key feature of cystine, allowing it to form strong bonds with other cystine molecules and participate in the formation of protein structures.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe the formation of cystine. While cysteine can be converted to other forms, such as homocysteine, the formation of cystine involves oxidation, not reduction.
* **Option B:** This option is incorrect because it suggests that cystine is formed through a process that does not involve the loss of electrons. While some amino acids can be formed through reduction reactions, cystine is formed through oxidation.
* **Option C:** This option is incorrect because it suggests that cystine is formed through a process that involves the gain of electrons. While some amino acids can be formed through reduction reactions, cystine is formed through oxidation.

**Clinical Pearl / High-Yield Fact**
Cysteine and cystine play important roles in the formation of protein structures, particularly in the context of keratin and collagen. The disulfide bonds formed between cysteine residues are critical for the stability and strength of these proteins.

**Correct Answer:** D.