I'm not exactly sure what you're asking, but I'll try.
The answer is "compro" because it's past tense. I should know, I speak both Spanish and English.
Answer:#2: Ellos "llegaron" tarde hoy.
#3: Ella esta navegando por el oceano.
#4: Yo estoy devolviendo mis libros a la biblioteca.
For me I would put:
Question 17: Ir a un concierto.
Because you don't just go to a concert in your free time.
Question 18: 3124
no puedo ir al cine
