What is the film-forming ingredient in most nail enamel?

The film-forming ingredient in most nail enamel is nitrocellulose. This compound is a modified form of cellulose that has been chemically treated to make it more soluble, enabling it to form a durable, glossy film when applied to the nails. Nitrocellulose helps create the distinctive hard surface characteristic of nail polish, contributing to its durability, shine, and overall appearance after application.

In the context of nail enamel formulations, other potential ingredients such as polyurethane or silicone can be present but are not primarily responsible for forming the film that provides the finish. While resins do play a role in contributing to the overall performance and adhesion of nail enamel, nitrocellulose remains the most prevalent film-forming agent in traditional formulations, making it the correct answer.