Answer: fueron
Explanation:
In this sentence, the verb that makes the most sense is fueron.
Ellos fueron a un concierto anoche. A verb is an action word, it describes an action, state, or occurrence. This sentence says they went to the concert last night.
The verb fue has six conjugations, yo fui, tú fuiste, él/ella fue, usted fue, nosotros fuimos, vosotros fuisteis, ellos/ellas fueron, and ustedes fueron. In the given sentence, we use the subject pronoun ellos, so the correct verb conjugation is fueron.
