What element of the hair structure is primarily responsible for its strength?

The strength of hair is primarily derived from the cortex. This middle layer of the hair shaft contains tightly packed cells and keratin, which is a protein that contributes significantly to the hair's overall durability and tensile strength. The cortex also holds the pigment that gives hair its color, and its structure plays a critical role in how hair responds to styling and chemical processes.

The cuticle, while important for protecting the inner layers of the hair and contributing to its overall appearance, does not provide the same level of strength. The medulla, although present in some hair types, is often absent in fine or thin hair and does not contribute to strength. The follicle is where hair growth occurs and supports the hair, but it is not a part of the actual hair shaft itself, which is where the strength resides. Thus, the cortex is the correct answer, as it is the primary contributor to the structural integrity of hair.