Can a single person apply for funding?
Yes, but the odds of being accepted are much lower. A startup is too much work for one person.
Do we have to be US citizens?
No, as long as you can get here for at least three months. We've funded many startups founded by non-citizens.
Can you get us visas?
No, sorry, we don't do that. You'll have to figure out visas for yourself. If you know people from previous YC-funded companies who came from outside the US, we suggest you ask them for advice. They understand the options better than we do.
While I do believe that successful business can be created by a single founder, I don't think that this is an issue when you're searching for someone to invest in your business. Anyway, I wish you good luck.