Final answer:
Collagen is best defined as a protein that forms connective tissue, providing structural support to various body tissues and making up 25% of protein content in mammals.
Step-by-step explanation:
Collagen is best defined as B. a protein that forms connective tissue. Collagen fibers are flexible fibrous proteins that are essential for providing tensile strength to connective tissue, which includes various types such as connective tissue membranes that encapsulate organs and lines movable joints, as well as dense connective tissue which contains a multitude of these fibers for elasticity and protection. Connective tissue holds in place, connects, and integrates the body's organs and systems. The extracellular matrix of connective tissue, where collagen is found, is secreted by the cells and consists of a complex assembly of proteins, including a significant amount of collagen, which makes up about 25% of the total protein content in mammals. It is distributed throughout the body, serving as a packing and binding material for the organs, and is a major component of the extracellular matrix (ECM).