Right answer:
estás
Since I am the one who hear this question, the correct answer is estás because this word comes from the verb estar. So estás is the conjugation of the verb estar for the second person singular in the simple present. Since this question is being addressed to me, the speaker must say: ¿cómo estás? (How are you?) and my response might be: Estoy muy bien? (I am very well).