Which property of hair determines its resistance to hair color?

The correct answer is porosity, as it is a crucial property of hair that affects how hair absorbs and retains color. Porosity refers to the hair's ability to absorb moisture and other substances, including hair dye. High porosity means the hair cuticle is raised or damaged, allowing for easier penetration of color molecules, which typically results in the hair taking color more readily and often fading more quickly. In contrast, low porosity hair has a more tightly sealed cuticle, making it less receptive to color. Understanding porosity helps in selecting the right coloring method and products that ensure optimal results on different hair types.

While other properties like elasticity, density, and diameter influence how hair behaves and styles, they do not directly determine how effectively hair can hold or resist hair color. Elasticity indicates how much the hair can stretch and return to its original shape, density relates to the number of strands in a specific area, and diameter refers to the thickness of individual hair strands. Each of these properties plays important roles in hair care and styling but does not specifically determine color absorption and resistance.