What type of nail product is used to smooth out and fill in minor depressions in a client's nail?

The correct answer is ridge filler, which is specifically designed to address minor imperfections in the surface of the nail. Ridge fillers work by creating a smooth surface over the uneven areas, helping to conceal ridges, bumps, and depressions. The formulation of a ridge filler often includes ingredients that can help to improve the overall appearance of the nails while providing a base that ensures better adherence for nail polish applied on top.

In contrast, nail polish is primarily used for coloring and may not effectively even out the surface of the nail. Nail strengtheners are formulated to enhance the strength and durability of the nails but do not focus on filling in depressions. Cuticle oil, while beneficial for moisturizing and maintaining the health of the cuticles, does not serve any purpose in smoothing or filling in any flaws in the actual nail surface. Therefore, ridge filler is the most appropriate product for achieving a smooth finish on nails with minor imperfections.