For Question 15, you would use "volaban" because it is in the "ellos" form.
(3rd person plural: ending -aban; the sentence is:
Los pájaros volaban sobre los árboles. )
For Question 16, the answer would be "menos", because "tonto" means dumb. Tigers or "tigres" are not as dumb as a burro.
For Question 17, you would use "trabajaba" because it is the correct way to write it in the "yo" form.
It's first person singular.
For imperfect, you generally add -ba- and another ending:
yo ... -ba
tu ... bas (e.g . tu volabas, tu trabajabas)
ellla ...-ba
nosotros ... -bamos
ustedes/ellos -ban
