I don't see that "what" would be a wrong choice here. It may be used as well as "which" in normal english to mean the same thing here.
How do you know when "Qual" means "what" and when it means "which"?
For example, "Qual é a minha janela?" means "Which one is my window?", whereas "Qual é a ocupacao dela?" means "What is her occupation?"
This discussion thread definitely helped, especially the Youtube video link provided by r_i_l_e_l_y. Although the video is about the difference between "Que" and "Cual" in Spanish, it seems to also apply to BP. For example, in both of the sentences I provided above as examples of "qual" meaning what or which, in both cases there is a limited number of possible answers, which according to the video is one of the usage rules about when to use "Qual" vs. "Que".