Which of the following is considered a common chemical used in hair styling?

Hair gel is considered a common chemical used in hair styling because it is specifically formulated for achieving various hair styles, providing hold, shape, and support to the hair. Hair gels often contain polyvinyl pyrrolidone (PVP) or similar polymers, which give the gel its distinctive texture and styling properties. Additionally, hair gel can come in different formulations that cater to various hair types and styling needs, making it a widely used product in the beauty and grooming industry.

In contrast, while onion juice and vinegar may have their respective benefits for hair health, they are not typically classified as styling products. Onion juice is often mentioned in the context of promoting hair growth due to its nutrients but is not a styling agent. Vinegar, particularly apple cider vinegar, is sometimes used as a hair rinse for its clarifying properties, but it does not provide hold or styling like hair gel does. Water, while essential for grooming and maintaining hair moisture, is not a chemical specifically designed for styling purposes.