The correct option is B had been left
The correct answer is ‘had been left’. Since this sentence is in the passive voice, we need to highlight that the object is being acted upon by the doer. Hence, the object ‘a special key’ appears first in the sentence. It is followed by the helping verb ‘had’, form of the ‘be’ verb ‘been’, the past participle of ‘leave’ which is ‘left’, the preposition ‘by’ and the subject ‘mother’.