Which of the following is the primary purpose of XML in Android app development?

The correct option is Define the app layout and design.

In Android app development, XML (Extensible Markup Language) is primarily used to define the user interface (UI) layout. Layout files written in XML describe the structure of the UI elements, such as buttons, text views, and image views, and how they are arranged on the screen.

The final answer is $\boxed{\text{Define the app layout and design}}$