I am a computer Engineering student and i want to choose my path as soon as possible. Will I be able to develop anything using C#?
Windows Phone 7's entire development platform is built on .NET and C#, so yes you can but that would be essentially your target mobile OS. Java will get you Android, and iPhone more or less requires Objective-C. Of course, you can get around each of these to some extent but by and large if you choose to learn C# you'll be targeting Windows Phone 7.
If you want to become directly valuable to a company as a mobile developer, learn Java or Objective-C. C# is fine, but Android and iOS have a much bigger market share than Windows Mobile 7.
Becoming an expert in any of those languages will be a good long-term career move.
No. You can use C# only for Windows Mobile. Java only works in the form of the Android API (which is somewhat similar to J2SE) on Android, and in the form of J2ME on Blackberry, Windows Mobile, and Android.
If you want to write mobile applications, you'll need to decide what platforms to write for, and learn the languages and APIs of each one of them. There are a few high-level languages that run on most mobile platforms (like Adobe AIR), but depending on your app, that's not an appropriate option.
Different mobile platforms require different languages, and future platforms will most likely require languages that don't even exist yet.
Your best bet is to pick one language / platform to focus on now, and expect to have to learn other languages / platforms throughout your career. Indeed, the chances are that you'll do other kinds of development, in addition to mobile computing development.
(For the record, I've been in IT for 30+ years, and so far I've used over 20 programming languages ... and encountered many others. You just deal with it.)
A lot of answers here indicate that c# is windows mobile / CE only; that is incorrect. In addition to targetting windows mobile and phone 7, c# can be uses to target iPhone via MonoTouch, and Android via MonoDroid. So that is the main contenders really.