Which substance is known as a solvent?

The correct choice identifies a solvent as a substance that dissolves other ingredients. In the context of esthetics, solvents are essential for creating solutions where active ingredients can be evenly distributed, allowing them to penetrate the skin effectively. For instance, when formulating skincare products, a solvent can help mix oil and water-based components, ensuring a homogenous product that can deliver its benefits more efficiently. This action is crucial for the efficacy of many cosmetic and therapeutic applications, as it facilitates the desired interactions between the formula's constituents.

Other options do not define a solvent's role. The first option refers to color additives, which serve a different purpose, while the third option pertains to preservatives that extend product shelf life, and the fourth option describes thickeners that change the viscosity of a solution. Each of these ingredients has its specific function that does not involve dissolving components in the way that solvents do. Thus, the understanding of solvents is integral to the formulation and effectiveness of various esthetic products.