Correct Answer: When was the mistake found?

Solution : The correct choice is the fourth option.

Passive voice is the voice in which the object experiences an action rather than the person who performs the action. Hence, the object in the active voice becomes the subject in the passive voice. Passive voice reverses the order, making the object of the active sentence the subject of the passive sentence, and uses a form of the verb "to be" along with the past participle of the main verb. "When was" introduces the passive construction and shifts the focus to the mistake (the object in the active sentence).

The structure of the passive voice sentence is: question word (when) + helping verb (was) + subject (the mistake) + main verb (found)?

Therefore, the correct answer is: When was the mistake found?