It depends on the context. If youre asking 'what is it?' then you would use cosa but due to the pecularites of italian you would use qual in certain questions where you would use 'what' in english such as asking 'what is your phone number which would be "qual e il tuo numero di telefono?". Its just something you have to remember. So i think in this instance "which is it?" would be the normal english translation without any context.