Who's that boy ? 用什么回答？It is …还是 He is…

yes she is
已知的人用she或he，未知的用it

回答yes,she is

Who's that boy? He is....

Is this your mother? Yes, she is.

因为原文中已经表明了提问对象的身份了，所以不用it is。

Who's that boy ? 用 He is…
Is this your mother ？用 yes，she is

it 用来表示"物" 它
he she "表示人" 他/她

Who's that boy ? He is...
Is this your mother ? yes she is
He是代表男型"他" She代表女性"她"
it 是代表动物"它"