The correct answer to complete the blank is:
-Trabajaron.
Mis hermanos trabajaron en un restaurante durante dos meses.
Explanation:
The key thing to solve the question is the tense of the sentence.The sentence is in past tense.Mis hermanos (my brothers) are a plural sustantive.The conjugation options are:
Trabajé (past tense but for singular person)Trabajamos (past tense but for nosotros (we))Trabajaron (correct. because is the right conjugation for they (mis hermanos))Trabajaste (past tense but for singular person)