Right answer:
a. salieron
Salieron is the conjugation of the verb salir that matches the third person singular in the preterite tense. The preterite tense is one of two simple past tenses. It is used to describe actions that took place or were completed at a certain point in the past. Finally, this sentence can be written as follows:
Al final, ellos salieron a comer almuerzo en el restaurante.