**Core Concept**
Collagen is the most abundant protein in the human body, forming a structural framework in various tissues such as skin, bone, tendons, and ligaments. It is a type of fibrous protein composed of amino acids like glycine, proline, and hydroxyproline, which provide its unique triple-helix structure.

**Why the Correct Answer is Right**
Collagen is found in skin, bone, tendons, ligaments, and cartilage, which are all connective tissues. The triple-helix structure of collagen molecules provides strength, elasticity, and flexibility to these tissues. In skin, collagen fibers are arranged in a specific pattern to provide tensile strength, while in bone, collagen acts as a scaffold for mineral deposition. In tendons and ligaments, collagen fibers provide elasticity and resistance to stretch.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because collagen is a key component of skin, which is the largest organ in the human body and provides a barrier against external factors.
**Option B:** This option is incorrect because collagen is found in bone, which is a hard, calcified tissue that provides structural support and protection to the body.
**Option C:** This option is incorrect because collagen is present in cartilage, which is a flexible connective tissue that covers the ends of bones and joints.
**Option D:** This option is incorrect because collagen is not typically found in muscle tissue, which is composed of skeletal muscle fibers that are responsible for movement and contraction.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that collagen production decreases with age, leading to wrinkles, joint pain, and osteoporosis. Maintaining a healthy lifestyle, including a balanced diet and regular exercise, can help promote collagen production and prevent these age-related conditions.

**Correct Answer: D. Muscle tissue**