Final answer:
Collagen fiber is the connective tissue fiber that is strong and difficult to stretch, providing tensile strength and resistance to tearing.
Step-by-step explanation:
The connective tissue fiber that is strong and difficult to stretch is collagen fiber. This type of fiber provides tensile strength to connective tissues, making them resistant to tearing and separation from surrounding tissues. Collagen fibers are a key component of both dense regular and irregular connective tissue. Dense regular tissue, found in ligaments and tendons, has fibers that are parallel and provide strength in one direction, making the tissue strong and less stretchy. On the other hand, dense irregular connective tissue has fibers arranged in a random pattern, giving it strength in all directions. Collagen gives fibrous connective tissue its durability and is essential in areas of high stress, such as the dermis of the skin.