**Core Concept**
Collagen is a major structural protein found in the extracellular matrix of connective tissue, playing a crucial role in providing strength, elasticity, and firmness to tissues such as skin, bones, tendons, and ligaments. It is the most abundant protein in the human body, accounting for approximately 25-35% of total protein content.

**Why the Correct Answer is Right**
Collagen is synthesized in the endoplasmic reticulum and secreted as a precursor molecule called procollagen, which is then processed by enzymes to form the triple-helix structure of collagen. This unique structure is stabilized by hydrogen bonds between the amino acid residues, particularly glycine, proline, and hydroxyproline. The correct answer highlights the importance of hydroxyproline in collagen synthesis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because collagen is not a type of elastin, which is another important protein found in the extracellular matrix, responsible for elasticity in tissues.
**Option B:** This option is incorrect because collagen is not a type of keratin, which is a protein found in hair, nails, and the outer layer of skin, providing structure and rigidity.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that collagen synthesis is dependent on vitamin C, as it is necessary for the hydroxylation of proline and lysine residues, which are critical for collagen stability. Deficiencies in vitamin C can lead to conditions such as scurvy, characterized by impaired collagen synthesis and connective tissue damage.

**Correct Answer: C. Collagen is rich in the amino acid hydroxyproline.**