Which of the following is a common ingredient in deep conditioning treatments?

Deep conditioning treatments are designed to penetrate the hair shaft and provide intense moisture and nourishment, and one of the most effective ingredients for this purpose is keratin. Keratin is a fibrous protein that constitutes a significant part of hair, and it helps to strengthen and restore the hair's natural structure. By incorporating keratin into deep conditioning products, it can replenish lost protein, improve hair elasticity, reduce frizz, and enhance shine, leading to healthier-looking hair.

While other ingredients such as sulfur, sodium chloride, and alcohol may have their uses in hair care, they do not serve the primary moisturizing and restructuring roles that keratin does in deep conditioning treatments. Sulfur is often associated with dandruff treatments, sodium chloride is commonly used as a thickening agent or preservative, and alcohol can often be drying and may strip moisture rather than add it. Therefore, keratin stands out as the key ingredient that aligns with the goals of deep conditioning.