As a matter of language, it isn't correct in grammar or word choice. This would be better:
"I am an intermediate level C programmer."
This is not a good interview answer, though, because there are no established standards and "intermediate level" doesn't tell the interviewer anything useful.
As an interview answer I like Edd's answer--"I have five years experience programming in C."
Even better would be a short answer that tells the interviewer HOW you used C and WHAT you accomplished with it; for example,
"I used C to write the lexical parser for my company's internal scripting language..."
"I used C in an embedded system when I rescued a project by taking over for an engineer who left suddenly..."