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.
Rojo azul negro verde morado naranja
1. Yo conoci al Presidente de los Estados Unidos.
2. Ellos conocian al docotor mas famoso del mundo.
3. Nosotros sabiamos (with accent on the i) que nos metimos en problemas.
4. Tus sabias (with accent on the i) que esa comida no era para ti.
5. Yo quiero unas papas con una hamburgesa.
6. Ella queria (with accent on the i) que la llevaras a comer.
7. El no queria (with accent on the i) ir al baile.
Hope this helps :)