Which product is primarily used to protect nails from stains?

The base coat is primarily used to protect nails from stains. It forms a barrier between the natural nail and the colored polish, preventing pigments from the nail polish from seeping into the nail and causing discoloration. This layer not only helps in maintaining the natural color of the nail but also promotes better adhesion of the polish, extending the wear time of the manicure.

The other options serve different purposes. Top coats are usually applied over nail polish to provide a shiny finish and enhance durability, but they do not have the same protective function against staining as a base coat. Nail polish remover is used to dissolve nail polish rather than protect the nails, and cuticle oil primarily serves to moisturize and nourish the cuticle area rather than act as a barrier against staining. Thus, the base coat plays a crucial role in nail care and protection.