Final answer:
The correct possessive adjective to complete the sentence is 'leurs'.
Step-by-step explanation:
The correct possessive adjective to complete the sentence is leurs, which means 'their' in French. In this case, 'leurs' is used because the noun frères (brothers) is plural. When referring to a plural noun that belongs to the third person, we use 'leurs'.
Learn more about Possessive adjectives in French