The right answer is prefirieron
Los muchachos prefirieron ir al parque cuando eran niños, pero ahora les gustan jugar deportes.
This sentence translates:
The boys preferred to go to the park when they were children, but now they like to play sports.
The word prefirieron is the conjugation of the verb preferir for the third-person plural in the preterite tense. In Spanish, the preterite tense (el pretérito) is used to describe actions completed at a point in the past.
