What design principle should be applied when creating apps intended for use on various devices?

When creating apps intended for use on various devices, responsive design is the most appropriate principle to apply. This approach allows the app's layout and elements to adjust fluidly to different screen sizes and orientations, ensuring an optimal viewing experience across a wide range of devices, from desktops to tablets and smartphones.

Utilizing responsive design ensures that content is accessible and user-friendly regardless of the device being used, which is crucial in today's multi-device environment. It typically involves using flexible grids and layouts, along with media queries that modify styles based on the characteristics of the device. This technique allows for a seamless transition of user experiences, improving usability and increasing engagement.

While adaptive design can also accommodate various screen sizes by creating distinct versions for different devices, it often requires more development effort and maintenance. Mobile-first design emphasizes designing for mobile devices initially, but it doesn't inherently provide the flexibility that responsive design offers across all devices. Static design, on the other hand, would not be effective at all for accommodating the variety of screen sizes that users may employ.