Ответ:

  1. Whose desk is that? — It’s mine. (b)
  2. When is your birthday? — The thirteenth of May. (e)
  3. Whose books are these? — They’re ours. (f)
  4. Are these his pencils? — No, they’re hers. (d)
  5. How old are you? — Thirteen. (c)
  6. Is that your calculator? — No, it’s his. (a)