The correct answer is D. Llamaba because he's talking in the past tense
La respuesta correcta es D. Llamaba porque está hablando en pasado.
I believe the answer is "un".
I remember taking a test like this, and I am 99% sure that was the right answer. Also, when you say it out loud, it makes more sense (If you have heard a bit of Spanish before).
I hope this helps you! I tried my best :)