Answer: The answers are:
1. Cuál.
2. Qué.
3. Qué.
4. Qué.
5. Cuáles.
6. Qué.
7. Qué.
8. Cuál.
9. Cuál.
10. Qué.
11. Qué.
12. Cuál.
13. Qué.
14. Qué.
15. Cuáles.
16. Qué.
17. Cuál.
18. Qué.
19. Qué.
Explanation: Just to elaborate a little on the answer, it can be added that in this exercise they are asking you to choose between the interrogative pronoun "cuál" (which one) and the interrogative adjective "qué" (what). The pronoun is used to inquire about the identity of a person or group of persons or things. For instance, in option 8 they are asking "Which one is the best?" and in option 15 they are asking "Which students study more?" The adjective is often used to inquire about the type or the category that a person or a thing belongs to. For example, in option 11 they are asking "What bottle of wine do you prefer?" Like the interrogative pronoun, it also asks for the identity, as in "What is this?" (option 7).