Final answer:
The appropriate preterite verb is 'fascinó'.
Step-by-step explanation:
The appropriate preterite verb in the sentence is fascinó.
When we use the preterite tense in Spanish, it indicates an action that was completed in the past. In this case, the verb fascinó shows that the brother was fascinated by something in the past.
For example:
Mi hermano se fascinó mucho con la película que vimos ayer.
Learn more about Preterite verb in Spanish