You can use the existing Telerik UI for Windows Phone 8 and create a Windows Phone 8.1 app. This can be done with the help of Windows Phone Silverlight 8.1 which "gives Windows Phone 8 developers access to some of the newest features available in Windows Phone Store apps without significantly modifying existing Windows Phone 8 code". This however means that when you submit the app to the store, it will not adapt for the layout and size of the tablet.
If you want your app to adapt automatically, then each component must have a phone and tablet counterpart making it universal.
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