In one respect, it doesn't matter. As you build a career in programming, you're certain to encounter C, C89, C99, C++ in some variant forms, C#, Java (as a quasi-variant of C) and Objective-C (as another quasi-variant) The goal, in my opinion, would be to establish yourself as a competent programmer first, and then any language you see, including Fortran, Cobol, or Basic, will be easy to read and understand, and therefore easy to adapt to.
In that sense, an early focus on any one of those languages is fine, as long as you're prepared to adapt to the other languages as they come along.
So I say this: If you're making this a focus of academic study, then choose the most popular standard C variant in use today, and do your learning while adhering strictly to the standard set. Following that, make an analysis of the differences between your choice, and the differences.
If this is a professional focus, you're going to want to learn C# and Objective-C and know them both cold, eventually, unless you're maintaining code. And if you're a smart professional, you're also becoming expert in PHP, Python, Ruby, Visual Basic (y'know, just. in. case.) etc, etc, etc...