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!"