Among the following, which product is used for smoothening the nail surface?

The product used for smoothening the nail surface is the buffer. Buffers are designed specifically for this purpose; they work by gently polishing the surface of the nail to eliminate ridges, enhance shine, and create an even texture. The use of a buffer helps prepare the nails for other cosmetic treatments and ensures a smoother appearance, which is essential for nail aesthetics.

Buffers come in various grits, allowing for both light smoothing and high shine finishes, and they can be used on both natural nails and artificial enhancements. This makes them a versatile tool in nail care, unlike other products listed, which have different functions. Nail polish, for instance, is used for coloring and adding shine, but does not inherently smooth the surface. Nail cleansers are meant for removing dirt and oils to improve adhesion, and cuticle removers focus on the skin around the nails rather than the nails themselves.