Question 16: The correct answer is la carta. You would put a letter (carta) in the mailbox (buzón).
Question 17: The correct answer is Fui estudiante. In this context, "fui" means "I was" as it refers to being a student.
Question 18: The correct answer is tuvieron. The correct sentence is "Ellos tuvieron un perro blanco," which means "They had a white dog."