Answer:
habréis decidido
Step-by-step explanation:
The correct sentence is ¿Vosotras habréis decidido cual mesero van a ocupar?
To determine which verb form is the correct one, you have to look at the subject of the sentence. As the subject is "vosotras", the verb must be conjugated in the pretérito perfecto compuesto of the 2nd person plural "habréis decidido". The pronoun vosotros/vosotras refers to more than one person.
The other options are also conjugated in the pretérito perfecto compuesto of the indicative mood, but what changes is the person in which they are conjugated.
-habré decidido: verb decidir conjugated in the pretérito perfecto compuesto of the 1st person singular (yo).
-habremos decidido: verb decidir conjugated in the pretérito perfecto compuesto of the 1st person plural (nosotros).
-habrán decidido: verb decidir conjugated in the pretérito perfecto compuesto of the 3rd person plural (ellos).
None of these persons (yo, nosotros, ellos) is the subject of the above sentence, so they all must be discarded. The subject is "vosotros" and the appropriate verb form to agree with it is "habréis decidido".