I notice that the words 'roupas' and 'usar' are frequently used together in Portuguese. You may of course use clothes, but having them on will sound more natural. Could it be that the correct translation of 'usar' in this context is 'have on' instead of 'use?' Hence in this case 'what clothes will she have on?'
When do you you use the singular for "roupa" and when the plural? In English the singular is cloth, which you can only hold or use for cleaning or making clothes from. You always wear clothes, never a cloth or a clothe. In Portuguese? I think I have made this mistake before. Can anyone explain?
What I have seen in various exercises and discussions leads me to believe that roupa refers to one set of clothes, or the clothes one happens to be wearing, whereas roupas refers to clothing in general, or one's wardrobe. Thus: "I have a lot of clothes." == Eu tenho muitas roupas.; "I need dry clothes." == Eu preciso de roupa seca.
In which situation would you use the Portuguese sentence? Since it does not use 'quais' I assume when you literally have nothing to put on, for instance you forgot to bring clothes on a trip or your house just burned down.
So, it is not just a matter of picking something from your wardrobe? Could it also be the press wondering about what a certain starlet might wear for a gala?