If they have the functions of interrogative adjectives, both form are just fine, although the use of CHE is more frequent than the use of QUALE, I believe the reason is the shorter and unchangable form of CHE in plural. (quale must be used as quali in the plural): Che musica ascolti? Quale musica ascolti? Che regali preferisci? Quali regali preferisci? The meaning of this che/quale is which/what BUT ONLY as ADJECTIVES followed by a noun. The other thing is if you want to ask "what are you doing", then what is a pronoun (not followed by a noun) and in italian that is only che/che cosa or cosa. Che or quale can be used as relative pronouns as well, but this is some other metter.

/quale/ is bound to a context of things among which to choose from, and the context is silent. /che/ is bound to an expressed (sometimes repeated) context q1a. Mi presti uno dei tuoi libri di fisica per favore? q1b.1. quale vuoi? q1b.2. che libro vuoi? q2a. Io conosco bene i piatti (dishes) cinesi. q2b.1. mi fa piacere, per me questi nomi sono misteriosi, quale mi consigli ? q2b.2. mi fa piacere, per me questi nomi sono misteriosi, che piatto mi consigli ? /che cosa/ is just a very generic context, without any hint on the kind of things. c1a. (some whispered words, quite difficult to understand) c2b.1. che cosa hai detto? c2b.2. che hai detto? Here 'quale cosa hai detto?' would grammatically correct, but quite unusual. c2a.1. che cosa voleva il tuo capo? c2a.2. che voleva il tuo capo? quite often /che/ in interrogative sentences is just a short form for /che cosa/.

You can use quale as which one. Che, cosa, che cosa have often the same mining, as what. "What are you doing?" could be "Cosa stai facendo?" "Che cosa stai facendo?" "Che stai facendo"

All kinds of use are possible. But the use of QUALE as QUALE COSA STAI FACENDO is wrong! QUALE means alternative that you know. If you don´t know the possibilities, than you should just use CHE, CHE COSA. So on, you can only use QUASE as it follows..."QUALE STRUMENTO HAI PRESO?" Never use as "QUALE COSE HAI FATO?". Just to end..."COSA HAI FATO" is the contraction form of "CHE COSA HAI FATO". Italians love to eliminate words. CIAO!!

Thankyou everyone for your help.