The person that asks the question wants to know if Mariela was the girl in the white house. The answer is a negation because it uses the word no, which is anexpression that means we use to contradict some or all of the meaning of a positive sentence. On the other hand, this is the correct answer because the word era is the conjugation of the verb ser for the third person singular in the imperfect, which is the case of the response.
