A nail primer is a substance that _____.

A nail primer is a crucial product used in the nail enhancement process, primarily to improve the adhesion of nail products to the natural nail. By using a nail primer, the surface of the nail is prepared to bond more effectively with subsequent layers, such as nail polish, gel, or acrylics. This enhanced adhesion is vital for the longevity and durability of the nail enhancement, preventing lifting and chipping.

The other choices do relate to nail care and enhancement in different ways but do not describe the primary function of a nail primer. For instance, changing nail color is typically achieved through nail polish rather than primer. Strengthening the nail is usually associated with specific treatments or hardening products designed to improve the nail's integrity. Reducing shine is a characteristic often related to specific preparation processes before applying nail products, rather than a primary function of primer itself. Thus, improving adhesion accurately captures the essence of what a nail primer does in the manicure process.