Final answer:
Elastic fibers, made of elastin, are the connective tissue fibers considered weak and easily stretchable, providing flexibility while being able to return to their original shape.
Step-by-step explanation:
The connective tissue fiber that is weak and easy to stretch is the elastic fiber. These fibers are composed of the protein elastin, which allows them to stretch to one and one half of their length and recoil back to their original size and shape. They provide significant flexibility to the tissues, such as those found in the skin, ligaments, and the walls of blood vessels. In contrast, collagen fibers confer strength and resistance to being torn, and reticular fibers form a network to support tissues and organs.
While there is looser connective tissue like areolar tissue that is more flexible, dense connective tissue includes types that have a greater quantity of collagen and thus greater resistance to stretching. However, even within dense connective tissue, there are types like dense regular elastic tissue that also contain elastin fibers, contributing to the tissue's elasticity.