Step 1: Identify the voice. 'The winning essay _______ written by a Primary 6 student' is passive voice — the essay (subject) received the action of being written. Step 2: Determine the tense. A 'winning essay' at the end of a competition is a COMPLETED past event. Past tense is needed. Step 3: Form the passive past. Passive past = 'was' + past participle for a singular subject. 'The winning essay was written'. Answer: was