3) te
4) la mochila es de el niño
5)a vosotros os gustan comer mariscos
the answer is A. Imperfecto progresivo.
The progressive imperfect is used to express something that was taking place in the past. It is formed using the verb to be in past (in this case: estaba) plus the verb ending in ing (ando, endo, in this case: cantando). It is mainly used for: on-going actions that were happening in the past.
El verano pasado yo fui de vacaciones a la playa
