Que le (m)olesta a tu hermano? los perros le (m)olestan a mi hermano. los perros (m)olestan a mi hermano.

The correct answer is:   [A]:

        →     " Los perros le (m)olestan a mi hermano. "

Note:  This is a "multiple-choice" question — with 2 (two) answer choices provided.   Among these 2 (two) answer choices:

     →   Answer choice:  [A]:  " Los perros le (m)olestan a mi hermano. "  ;

               →   is, in fact, the correct answer choice.

Note that the Spanish question being asked:

       →  " ¿Qué le (m)olesta a tu hermano? "  ;

translates (in English)  to:

       →  " What {bothers/disturbs/annoys] your brother? "

The 2 (two) answer choices being presented translate to:

          " Dogs {annoy/bother/disturb} my brother. "

 Or:   " The dogs {annoy/bother/disturb} my brother. "

Note:  In this case, using proper Spanish grammar, you must include the "indirect object Spanish pronoun" — which is:  " le " ;

→  which corresponds to the "third person, singular, form" ;

→ {that is:  " Ud./usted " / " él "  / "ella" /  "single person first name — (male or female)" ; / or:  "any singular Spanish noun (regardless of masculine or feminine form)" — of the "Spanish indirect object pronoun."

Note that this 'Spanish grammar rule' holds true even when the actual indirect object is stated in the sentence (both in written Spanish as well as spoken Spanish — thus indicating that:  

"Answer choice:  [B]:  " Los perros (m)olestan a mi hermano." ;

                          →   is incorrect .

For this question:  Answer choice:  [A]:

    →    " Los perros le (m)olestan a mi hermano. "  ;

         →  is the correctly written answer.

Note that the "indirect object" is:  "hermano" {"mi hermano"} ;

    or, "brother" {"my brother}"  —  (in English) ;

           →   which is a 'singular noun' ; in the 'third person' form.  

        →     As such, " le " — is the appropriate — (and correct!)

                 —  "Spanish indirect object pronoun" to use.

The correct answer is:  

  →   Answer choice:  [A]: " Los perros le (m)olestan a mi hermano. "

Hope this answer is of help to you!

   Best wishes in your academic endeavors

          — and within the "" community!