As an interviewer I would really rather not sit and watch someone write C on a whiteboard. Use whatever language you feel like you can skip to the interesting parts of algorithms. Even pseudo code could work.
As an interviewer, it gives me a chance to step back from noticing syntax errors and lets me focus on how the candidate thinks, reasons and explains her way through problems.
As an interviewee, I prefer pseudo code because I don't have to worry about syntax or minute details of a language. Instead, I can just focus on communicating how I'm working through the problem.
Personally I choose Python given the option: easy to write, slices and comprehensions generally make algo question answers very terse.
One interviewer laughed and asked me if I wanted to switch languages, right off the bat. In retrospect, that was gracious. He said I was probably not going to finish in time, but.. if I really wanted to.. I could give it a try. I kinda froze.
The last interview was more of looking at sample code / code review, and get this: it wasn't even in C, it was C++. Not what I expected! Turns out I should've been studying that, too. They are certainly similar, but it's just another curve ball.
But be prepared to read a lot of solutions to problems written with for loops, do-while loops, and other constructs that are not pythonic and don't translate as well to pythonic code.
In a way it's good, because it forces you to re-implement the algorithms in a way that makes sense to you, thereby increasing your chances of actually learning and retaining the information.
That said, it helps to use a language in the company's stack.
BTW, totally serious: I've had to write the code in binary several times. I know a few opcode representations.