Depends- did you get a C on an upper-level physics course? or general Bio? or studio art? Having a lower grade in a prerequisite matter more than a random elective. But I have seen people get a C in a perquisite class, like organic chemistry, and still get into great dental schools. As long as it’s not a trend, or an average, getting a lower grade on a single class would no way be the sole reason for getting rejected. Schools use a more holistic process.