Tendons are the connective tissues that attach muscles to bones, allowing for movement and strength. These tendons are composed of long fibrous proteins that provide the necessary strength and flexibility.

Option 1: Fibrin is a protein involved in blood clotting, not the construction of tendons.

Option 2: Collagen is the correct answer. Collagen is the main protein found in tendons and provides their strength and structure. It is a tough and fibrous protein that gives tendons their ability to withstand tension and stress.

Option 3: Elastin is also a fibrous protein found in connective tissues, but it is responsible for elasticity and stretching, not the strength and structure that tendons require.

Option 4: Cellulose is a complex carbohydrate found in plant cell walls, not a protein found in tendons.

Therefore, the correct answer is option 2: Collagen. It is the primary protein component of tendons, providing the necessary strength for muscle-to-bone connection.