**Core Concept**
Odontoblasts are specialized cells in the dental pulp responsible for the formation of dentin, a hard, calcified tissue beneath the enamel of the tooth. The secreted protein plays a crucial role in the development and structure of dentin.

**Why the Correct Answer is Right**
Collagen is the primary protein secreted by odontoblasts, accounting for approximately 90% of dentin's organic matrix. Type I collagen is the most abundant form, which forms a network of fibers that provide strength and rigidity to dentin. The collagen fibers are densely packed and arranged in a specific pattern, giving dentin its unique structure and properties. The presence of collagen in dentin is essential for its mineralization and overall tooth development.

**Why Each Wrong Option is Incorrect**
**Option A:** Chitin is a polysaccharide component of exoskeletons in arthropods and fungal cell walls, not a protein secreted by odontoblasts.
**Option B:** Keratin is a structural protein found in skin, hair, and nails but is not associated with tooth development or dentin formation.
**Option D:** Elastin is an elastic protein found in connective tissue, particularly in the skin and arteries, and is not secreted by odontoblasts.

**Clinical Pearl / High-Yield Fact**
The unique structure and composition of dentin, including the presence of collagen, provide a scaffold for the deposition of minerals, ultimately leading to the formation of a hard, calcified tissue.

**✓ Correct Answer: C. Collagen**