Answer:
1. She
2. He
3. It
4. It (It if it's one car, they if it's multiple)
5. They
6. It
7. We
8. He
9. They
10. They (Since we are not given the gender)
11. It
12. They
13. They (It if it's one book, they if it's multiple)
14. We
15. They
16. They
17. It
18. It
19. It