What is the main element in hair responsible for its strength?

The main element in hair responsible for its strength is keratin. Keratin is a type of fibrous protein that constitutes the primary structure of hair, skin, and nails. This protein provides rigidity and resilience, which are essential for hair's ability to withstand various environmental factors and styling practices.

Unlike collagen, which is primarily found in connective tissues and provides support and structure to skin and bones, keratin acts directly in hair's architecture. Elastin, while it offers elasticity to skin and certain tissues, does not play a significant role in hair strength. Calcium is a mineral that is important for various bodily functions, but it is not a structural component of hair. Thus, keratin's unique properties are what provide hair its strength and durability, making it the correct answer for this question.