Estábamos sirviendo would be the answer for the nosotros form in the imperfect progressive. For future reference, you should check out the Spanish Dict online, it has all of the verbs in every form and tense :)
3. Dónde
4. Qué
5. Dónde
6. Cuántas
7. Cuándo
8. El
9. Qué
10. Porqué
11. Cual
12. Ese
13. A qué
14. A quién
15. Qué
Hwooo hopefully I could help with this sorry I took long :)
Answer:
A. Su
Explanation:
Because Su literally translates to their. Tu means you/your and mi means mine/my.
If it were casas, then you just modify the pronoun to sus.
