The letter __________ by the secretary yesterday.

The correct answer is B: was written. This sentence is in the passive voice because the subject, 'The letter', is receiving the action of being written, rather than performing it. The phrase 'yesterday' clearly indicates that the action took place in the past. Therefore, we need to use the past simple passive form. The past simple passive is constructed using 'was' or 'were' (depending on the subject's number) followed by the past participle of the main verb. Since 'The letter' is singular, 'was' is the correct auxiliary verb. The past participle of 'write' is 'written'. Thus, 'was written' correctly forms the past simple passive voice. Options A ('is written', present passive), C ('wrote', active past simple), and D ('writes', active present simple) are incorrect as they do not fit the passive voice requirement for a past action.