Which product is commonly used as a base for mixing with other substances?

A solvent is commonly used as a base for mixing with other substances because it has the ability to dissolve other compounds effectively. In the context of esthetics and skincare, a solvent can be a liquid that helps to dissolve active ingredients, making them easier to incorporate into formulations such as creams, lotions, or serums. This property allows for better absorption into the skin, enhancing the effectiveness of the active ingredients in the final product.

Moisturizers, while important in skincare for hydrating the skin, are not typically used as a base for mixing other substances in the same way a solvent is. Exfoliants serve a different purpose by removing dead skin cells and improving skin texture rather than acting as a mixing agent. Serums are concentrated formulations meant to deliver specific active ingredients to the skin; they may contain solvents but are not themselves used primarily as a base for mixing other substances. Thus, a solvent stands out as the correct choice for the question regarding a base for mixing.