Which protein forms a network of fibers in the body to provide support for connective tissues?

The correct answer, reticulin, is a type of collagen that forms a delicate network of fibers to support the structure of various tissues, particularly in connective tissues like lymphoid tissues, bone marrow, and the spleen. This network helps maintain the shape and integrity of these tissues and provides a framework for cells and other components to attach and function properly.

In contrast, while collagen is indeed a major structural protein in the body, particularly abundant in skin, bones, and tendons, it is characterized by its strength and resistance to tension rather than the delicate framework characteristic of reticulin. Keratin, on the other hand, is the structural protein found in hair, nails, and the outer layer of skin, primarily serving protective functions but not contributing significantly to connective tissue support. Elastin provides elasticity and resilience to tissues like the skin and blood vessels, but does not form the same kind of supportive network that reticulin does for connective tissues. Thus, reticulin's specific role in providing a supportive network for connective tissues makes it the correct answer.