You use preterite to describe an action that took place and was fully done, Ex. José entro a la habitación, you can see John went into the room, he completed the action. On the other hand, imperfect tense doesn't specificate if the action was completed, Ex. José entraba a la habitación, it doesn't tell us if José fully entered the room.
There could be many answers.
For example:
Tomaron
Robaron
Tocaron
But I think the right answer would be....
Ayer, Roberto y Carlos gastaron 12.000 pesos.
