Which of the following substances is crucial for maintaining skin elasticity?

The correct choice is elastin, which plays a vital role in maintaining skin elasticity. Elastin is a connective protein found in the skin, blood vessels, and other tissues, allowing them to return to their original shape after stretching or contracting. This property is essential for maintaining the skin's firmness and resilience, as it helps the skin withstand various stresses and movements.

While collagen is also important for skin structure and strength, it primarily provides tensile strength rather than elasticity. Melanin and pheomelanin are types of pigments responsible for skin color and protection against UV radiation but do not contribute to the elasticity of the skin. Thus, elastin is the substance that directly influences the skin's ability to stretch and maintain its shape, making it crucial for skin elasticity.