Which of the following could be a condition that a veneer improves?

A veneer is a thin layer of porcelain or composite resin that is custom-made to fit over the front surface of a tooth, primarily for cosmetic purposes. One of the key functions of veneers is to enhance the appearance of teeth, making them look more uniform and aesthetically pleasing.

In the case of closing a diastema, which refers to a gap between two teeth, veneers can be applied to the adjacent teeth to effectively "close" the space. This provides an immediate visual change and can improve the overall smile by creating a seamless appearance. The strength and durability of the material used for veneers also ensure that the result is not only attractive but also functionally sound, allowing normal dental activities such as chewing and speaking without hindrance.

The other conditions mentioned, such as deep cavities, periodontal disease, or tooth extraction, generally require different types of dental interventions. For example, deep cavities would necessitate restorative procedures like fillings or crowns, while periodontal disease might require periodontal treatment to address gum health, and tooth extraction is a separate procedure entirely focused on removing a tooth rather than improving cosmetic appearance. Therefore, the improvement of a diastema by using veneers stands out as the most relevant condition that veneers can effectively address.