When asked a question which contains the phrase "Question word + が", the answer should be "Nが～". The particle が marks the subject of a sentence in an emphatic way. Therefore, in sentences which focus on the subject, it is が, not は, that is used to mark the subject.
A: 本が 二さつ あります。どちらが あなたの 本ですか。
There are two books. Which one is yours?
B: こちらが わたしの 本です。
This is my book.
A: どんな 家が いいですか。
What kind of house do you prefer?
B: 広くて、明るい 家が いいです。
I prefer a big, bright/sunny house.
Lesson 7-5「だれが 来ましたか。」
Who came?