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.
What the other guy said was right, I just want to add that if you see something “I drove many times” that would be imperfect. Anything in the past that was done more than once is imperfect
Example
“Yo manejaba muchas veces”
“Yo jugaba cada mes”
“Yo manejé una vez”
