Hi, Nadezhda! Generally speaking, English's default pronoun form is the accusative (objective) form. If there is no syntactic context that requires another form to be used and/or the answer is a standalone, English speakers use the accusative form. Thus, you'll never hear an English speaker respond with only "I"; there must be a syntactic reason to use the nominative (subjective) pronoun (e.g. "Am I the one you are asking?"). I hope this helps. Please let me know if I did not answer your question.

Grammatically, “I” would be correct. But this is an example of a sentence in which 99% of American English speakers would say “Me” because either (i) they don’t know better, or (ii) saying “I” sounds pompous/stiff/too formal. Another example: A: “Who’s there?” B: “It is I.” Everyone says “It’s me”, even though “It is I” is correct.

let me try my 2 cents: "I" seems a grammatically correct answer, however, if B changes his question a bit like this: is the "you" you're referring is "ME"? are you talking about "ME"? then dose "Me?" sounds a lot natural now?

I is a subject prounoun and it's used for the subject of the sentence. He, she, we, it, they and you are also subject pronouns. "Hugo and I work in the same office" - 'Hugo and I' are the subject. Me is an object pronoun and it's used for the object of the sentence. Him, her, us, it, them and you are also object pronouns (you can be both a subject and object pronoun). "Lisa bought Amanda and me a drink" - 'Lisa' is the subject, 'Amanda and me' are the object.

Why do we use "Me", not "I", in a case like this: "Why are you up so early?" "Me?" Good question! The French do the same: «moi?». I think it is because that is the way it has been traditionally answered for years, the common people having a penchant for "me" over "I". Also, it could be short for "are you talking to me?" If you had asked " why up so early, Sam?". Sam would remember his grammar and answer "I? Does her mean I?" Question for you: "It is I" surely implies, being equivalent, "I is it", but this is never said. Why?