Hello Lea, Both are "correct". They just have different subjects. Try answering the question, e.g.; Q) Which one is you? A) That one is you. Q) Which one are you? A) You are that one. (you can't say "That one are you".) This would be the only difference.. as far as I am aware. I hope it helped.

Thank you, Richard. I thought I mastered 'the verb be' grammar. Now when I practice them, I got confused still. :D

The form of the verb depends on the pronoun 'you', hence 'you are'. The form of the verb does not depend on 'which' or 'one'. When the sentence is in the interrogative form, we simply reverse the order of the verb and pronoun. http://learnenglish.britishcouncil.org/en/english-grammar/verbs/verb-be