In Flutter, custom responsive layouts can be achieved by defining breakpoints in the ________ function.

Custom responsive layouts in Flutter can be achieved by defining breakpoints in the 'build()' function. The 'build()' function is where the UI is constructed, and by strategically placing breakpoints within this function, developers can adapt the layout based on the device's characteristics, such as screen size or orientation. Understanding how to use breakpoints in the 'build()' function is crucial for creating responsive and adaptive Flutter applications.