Which verb form correctly completes this sentence? ayer, mis primos 15.000 pesos.

Preterite

Explanation:

The sentence translates as "Yesterday, my cousins 15,000 pesos".

A verb from the preterite verb tense, indicating a punctual event in the past.

We know for sure it's in the past because the sentence starts with 'Ayer' - "Yesterday".

We assume it's punctual because it involves something about money a money transaction is usually punctual, it's not a mood or a description of a situation.

Unless you have a specific verb to go with that sentence, the verb could be:

(to pay): pagar -> pagaron

(to lose): perder -> perdieron

(to lend): prestar -> prestaron

and so on...