Question

1. When's he been to Japan?
2. When has he been to Japan?
[Can we use #1?]

3. Why's he sad?
4. Why is he sad?
[Are both okay?]

5. Why's he been to Japan?
6. Why has he been to Japan?
[Are both okay?]

7. Where's he gone?
8. Where has he gone?
[Are both okay?]

9. Who's the book?
10. Who has the book?
[Is #1 grammatical?]

11. Who's been working at the factory?
12. Who has been working at the factory?
[Are both okay?]

Answer 1

1, 3, 5, 7, and 11 are used in casual speech. 9 is not used.

The others are correct in writing and speech.