Answer:
a pris : passe compose
avait pris : plus que parfait
Explanation:
Since you were giving to empty slot to fill, we can guess the tense will use an auxilary verb. Both above can be used. But I would suggest using the passe compose one since it's the most common.
Remember that here since you are using the auxiliary avoir the main verb pris (prendre) accords only with the direct object (here it is bus), when it is placed before the verb. Since it is not the case here it doesn't change.
