In WPF/XAML, styles can be defined at multiple levels: App.xaml (application-wide resources), inline directly on controls, and in resource dictionaries (which can be merged or defined at various levels). Option B (MainPage.xaml) is incorrect as a standalone answer - MainPage.xaml would use a resource dictionary section, but MainPage.xaml itself is not a style definition mechanism. The correct locations are resource sections, not arbitrary XAML files.