¿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 helped,
BioTeacher101