Final answer:
Collagen is the connective tissue fiber that is strong and difficult to stretch, which provides tensile strength and resistance to being torn. The correct option is A.
Step-by-step explanation:
The connective tissue fiber that is strong and difficult to stretch is collagen. Collagen fibers are the main component of dense connective tissue, which contains many collagen fibers that provide both elasticity and protection. Collagen gives connective tissue tensile strength, preventing it from being torn or separated from the surrounding tissues.
While elastin fibers, made of the protein elastin, offer flexibility and can stretch and return to their original shape, and reticular fibers form a supportive network, it is the collagen fibers that are most noted for their strength and resistance to stretching.