Which of these is the best way to repair a deeply furrowed nail?

Using ridge filler to even out a deeply furrowed nail is the best approach because it addresses the surface irregularities directly. Ridge fillers are specially formulated products designed to smooth out depressions and imperfections on the nail plate. When applied, they create a more uniform surface that can enhance the appearance of the nail, allowing for even application of nail polish if desired. This method not only improves aesthetics but also helps protect the nail from further damage by providing a smoother top layer.

Other methods, such as trimming the nail, may not provide a solution for the resulting texture. Applying a nail strengthener can be beneficial for strengthening the nail, but it does not specifically address the surface issues created by the furrows. Leaving the nail untreated would not provide any improvement or protection and could lead to further problems. Therefore, using ridge filler is the most effective and appropriate method for repairing a deeply furrowed nail.