Can I ask you something? Could I ask you something? You'll hear people use both of these in English. "Could" comes across are more polite. You can answer both of these with "Yes, you can." - so B, from your question. "May I ask you something?" is also a more polite way of asking the same question, and is what I tend to use myself. However, "May" can only be used for yourself. "May I ask you something?" is correct, but "may you fetch me a drink" wouldn't work. "May" can only be about yourself, "I". In that case you would say "could you fetch me a drink?"/"can you fetch me a drink?". "May I ask you something?" "Yes, you may." But you can avoid all of these by answering with "Yes, of course!" :p

It's B