To allow a variable to be accessible by a button in Unity, what must be its access modifier?

For a variable to be accessible by a button in Unity, it must be declared with a public access modifier. This is because Unity's serialization system and the Inspector interface only display variables that are public or serialized fields. When you make a variable public, it becomes accessible not just in the script itself but also allows other components and the Unity UI system, such as buttons, to interact with it.

Having a public access modifier enables other scripts and UI elements in your Unity project to reference and modify the variable, making it essential for functionality where you want the UI, like buttons, to control game logic or display data.

While private variables remain accessible only within the same class and cannot be interacted with by UI elements directly, protected variables are accessible only by derived classes. Internal variables are only accessible within the same assembly, which also does not meet the need for UI interaction. Therefore, using the public access modifier ensures proper access and interaction with UI elements in Unity.