The given answer "les beaux-frères" is the correct plural form of the noun "le beau-frères" in French. In French, nouns have different forms depending on their gender and number. "Le beau-frères" is the singular form, meaning "the brother-in-law." To make it plural, the article "les" is used, and the noun is changed to "beaux-frères." This means "the brothers-in-law." The plural form matches the gender and number of the noun, making it the correct answer.