Which of the following is considered a built-in layer in Unity?

In Unity, certain layers are predefined and are known as built-in layers, which provide a structured way for developers to sort and manage game objects in a scene. The "Default" layer is one of these built-in layers and is automatically available upon creating a new Unity project. It serves as a basic layer where most game objects are placed unless specified otherwise.

Using built-in layers like "Default" simplifies the organization of game objects for operations such as collision detection, rendering settings, and performance optimizations during gameplay. For instance, when setting up physics interactions or managing visibility through camera settings, the Default layer can be easily referenced.

Other options such as "Player," "Enemy," and "NPC" are not built-in layers in Unity. These options represent more specific categorization schemes that a developer might create to organize custom game objects according to their roles in gameplay. While you can define layers with these names to suit the needs of your project, they do not come as part of Unity's default set of layers at project initiation.