Question

is that your grangmother ? 肯定回答

Answer 1

肯定回答：Yes, it is. 否定回答：No,it isn't.
以“Is this/Is that"引导的一般疑问句回答时不能用this is 或 that is, 而应该用it is或it isn't.
至于Yes, she is貌似正确，实则不然，因为这是中式思维的体现，不能因为询问的对象是女性，就用she， 仍然需要用it. 因为在此it不是对应的grandma，而是对应疑问句中的that.
因此本题正确答案如开头所示，而且不能说Yes, it is my grandma 或者No, it isn't my grandma, 因为这样it 就指代(或对应)grandma了。

Answer 2

Yes, she is.

Answer 3

Yes,she is.

Answer 4

Yes, she is.