It's completely context dependent.
- Compared to PHP, C is low level;
- Compared to x86 assembly, C is high level;
- Compared to the instructions I used to construct this universe in which you reside, C is so high level you can barely see it.
It depends who you're talking to, and about what, as to what the answer is. C is a bit of a unique beast, though, because (as Mike pointed out in comments), it's the lowest-level of all general-purpose, architecture-independent programming languages, and these are the languages you're most likely to encounter in modern reality. So, in practice, you're likely to hear C being deemed "low level" unless you're talking to someone who fiddles bits in registers with a soldering iron for a living.