**Core Concept**
Collagen is a primary structural protein found in the extracellular matrix of connective tissue, providing strength and elasticity to various tissues and organs in the body. It is the most abundant protein in mammals and plays a crucial role in maintaining tissue integrity.

**Why the Correct Answer is Right**
Fibroblasts are a type of cell that produces collagen and other extracellular matrix components, but they themselves are not composed of collagen. Collagen is secreted by fibroblasts and then deposited into the extracellular space, where it forms a network of fibers that provide structural support to tissues. This process is essential for the development and maintenance of connective tissue, including skin, bone, and cartilage.

**Why Each Wrong Option is Incorrect**
**Option A:** Ligaments are fibrous connective tissue that connects bones to other bones, and they are rich in collagen fibers. Collagen provides the necessary strength and elasticity to ligaments, allowing them to withstand tensile forces.
**Option B:** Tendons are also composed of collagen fibers, which give them their strength and elasticity. Tendons connect muscles to bones and transmit forces generated by muscle contractions.
**Option D:** Aponeurosis is a type of flat tendon that connects muscles to bones, and it is also composed of collagen fibers. Aponeurosis provides a strong and flexible connection between muscles and bones.

**Clinical Pearl / High-Yield Fact**
Remember that collagen is not a cellular component but rather a protein produced by cells, such as fibroblasts, and deposited into the extracellular matrix.

✓ Correct Answer: C. Fibroblasts