If you want to develop android apps in .NET the perhaps usinf the Xamarin framework is the best option.
http://xamarin.com/[
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Alternatively, if you are not restricted to using .NET and you are acquainted with HTML and JavaScript then perhaps developing applications using HTML5/Javascript is also an option. But be aware that these applications will be web applications developed and they will have less capabilities than the applications developed in the Java(Android) and objective-C(iOS).