Im a college junior. Ive got fairly good experience with C++, C#, and Java SE. But I dont have any experience or knowledge in the web application side of things. I really want to add one technology to my skill set.

I donot know Php well. All I know is HTML. These are my options...

Should I go for Python or Ruby on Rails or Scala?? Please guide me.

They're all good. Since you're familiar with C# you might want to make your web app in C# using the MS MVC framework. Since you're familiar with Java, using Scala would be an excellent choice. I'd say these are your top choices.