Answer:
The right answer is, Encontraron.
* ¿Encontraron ustedes sus libros antes de la clase hoy?
Explanation:
Because the sentence refers to an action performed in the past tense without defining its end, which indicates a use of the preterito tense, therefore it is important to identify the person, number and gender to correctly conjugate the verb and be consistent with what is expressed in the sentence.
