What is the name of the highly elastic protein in connective tissues that allows them to stretch?

The highly elastic protein found in connective tissues that allows them to stretch is called elastin. This protein plays a crucial role in the elasticity and flexibility of various tissues such as skin, lungs, and blood vessels. Elastin allows these tissues to return to their original shape after being stretched or contracted, which is vital for normal function.

Unlike collagen, which provides strength and structural support, elastin's unique properties enable it to accommodate fluctuations in size and shape without losing integrity. Keratins, on the other hand, are more associated with protective roles in skin and hair, and fibronectin serves as an important glycoprotein in cellular adhesion and tissue repair, but neither of these proteins contributes to the elasticity required in connective tissues the way elastin does. Hence, elastic properties in various tissues are primarily attributed to elastin, making it the correct answer.