What type of stairs serve only apartments in a specific wing of a building?

The correct answer is "wing stairs." Wing stairs are designed to provide vertical access exclusively to specific apartment wings within a larger building. This setup allows residents to access their homes directly without having to navigate through shared or common areas, enhancing both security and convenience.

In contrast, other types of stairs, such as emergency stairs, are typically reserved for use during emergencies and may not provide direct access to resident apartments. Shared stairs serve multiple units or common areas, which does not align with the exclusive use of apartment access. Single stairs may suggest a solitary stairwell but do not specifically denote access to only one wing of apartments. The functionality of wing stairs directly relates to their design purpose in residential buildings, making them the most suitable answer.