¿qué están haciendo ustedes? - what are you (pl.) doing?
given the info above, our answer must contain the word "we".
we = nosotros. - following the conjugation rules, some examples of words in the correct "we-conjugation" would be:
comer (to eat) - comemos (we eat)
andar (to walk) - andamos (we walk)
estar (to be) - estamos (we are)
so, our conclusion consists in that our correct sentence must contain the word "estamos", so our answer is the first one:
¿qué están haciendo ustedes? - estamos haciendo nuestra tarea.
hope it ,
bioteacher101