I have developed a keyboard extension for iphone and submitted to Appstore. But it got rejected due to- 2.10: iPhone Apps must also run on iPad without modification, at iPhone resolution, and at 2X iPhone 3GS resolution.
Do I need to make universal App, or just make it scalable up to ipad and submit to App store?
You don't need to make it universal. It only needs to run on the iPad at the iPhone resolution.
SOURCE: WWDC sessions