Final answer:
The thin branching fibers that allow the skin, lungs, and arteries to stretch and recoil are made of the protein elastin, and they are known as elastic fibers.
Step-by-step explanation:
The thin branching fibers made of elastin that stretch and recoil, giving the skin, lungs, and arteries the ability to stretch and then return to their original shape, are known as elastic fibers. These fibers contain a high percentage of the protein elastin. One of the key characteristics of elastin is its ability to extend and then snap back to its normal length when released. Elastic fibers are an integral component of connective tissue and are prominent in parts of the body that require flexibility and elasticity, such as the skin, lungs, and arterial walls. The correct answer to the student's question is 3) Elastin.