Answer: The answer is Son
Explanation: It is the masculine form of "they are" which when added to the sentence means "My brothers are very hardworking."
Ustedes estudian en la biblioteca is the correct answer.
Spanish has verb suffixes for each pronoun, which is something English only has for third person singular (-s, or -es). So, this is how to conjugate the verb estudirar (=to study):
1. yo estudio 1. nosotros estudiamos
2. tu estudias 2. vosotros estudiais
3. el/ella/Usted estudia 3. ellos/ellas/Ustedes estudian
Estudian (present)
Estudiaron (past)
Me gusta correr.
No me gusta trabajar.
Me gusta más practicar deportes.
No me gusta nada bailar