What is what does b mean when a guy says it to a girl?

It's tough to say definitively what "b" means when a guy says it to a girl without more context. Here are some possibilities, ranging from platonic to potentially romantic, with links for further exploration:

To figure out the meaning, consider the guy's [https://www.wikiwhat.page/kavramlar/Typical%20Communication%20Style](typical communication style), their existing relationship, and the tone in which he says it. The best way to know for sure is to ask him directly!