Which of the following can cause nail discoloration?

Nail discoloration can indeed be caused by a variety of factors, and one of the most significant causes is fungal infections, staining from nail polish, or smoking.

Fungal infections often lead to discoloration, typically showing a yellow or greenish hue on the nails, and in more advanced cases, the nail may become thickened or distorted. Certain nail polishes, especially those that are not breathable or contain strong pigments, can stain the nails, leading to yellowing, especially when used frequently without a base coat. Smoking is another contributor, as the nicotine and tar can leave a yellow or brown stain on the nails and surrounding skin.

While moisture and humidity can affect the nails in terms of weakened structures or susceptibility to infections, they do not directly cause discoloration. Exposure to sunlight can lead to damage such as fading or drying but is not typically associated with nail color change. Cutting nails too short primarily affects nail health and comfort rather than color. Therefore, the factors listed in the correct answer specifically indicate direct causes of discoloration.