Final answer:
The connective tissue fiber that is easy to stretch is elastin, which provides flexibility to tissues and can significantly extend and return to its original shape.
Step-by-step explanation:
The connective tissue fiber that is weak and easy to stretch is elastin. The fibers made of elastin can stretch significantly beyond their original length and then return to their initial shape. This property of elastin provides flexibility to various tissues such as the lungs, the walls of large arteries, bladder, and elastic ligaments. While collagen fibers provide tissue strength and resistance to stretching, elastin facilitates flexibility and reticular fibers form a network to support the tissue structures. Therefore, the correct answer is b) Elastin.