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I am writing a detailed article on the same topic.
In brief, you can build a browser mobile application. This is actually a Web application optimized from mobile phones. For this, you create a regular ASP.NET application and use Mobile ASP.NET browser controls.
You can also use many tools out there to build mobile (non ASP.NET) applications including Adobe suites.
If you are building mobile apps (download and run on the mobile), you need to use an app framework. There is a separate framework for each device. For example, you will need to use Objective-C for iPhone, Silverlight for Windows Phone, and Android SDK for Android phones.
More details in this thread:
http://www.c-sharpcorner.com/Forums/Thread/141905/how-can-i-develope-web-application-for-iphone-and-andriod.aspx