The correct passive form of the given sentence is:

☀ The smouldering fire was fanned by the fast wind.

Let's break down the options:

1. The smouldering fire was fanned by the fast wind. (Correct) This sentence correctly uses the passive voice and conveys the same meaning as the original sentence, indicating that "the fast wind" performed the action of fanning "the smouldering fire."

2. The smouldering fire is fanned by the fast wind. (Incorrect) This sentence is in the present tense, which changes the meaning of the original sentence. The original sentence is in the past tense, describing a specific event that already occurred.

3. The fan was smouldered by the fast wind. (Incorrect) This sentence is in the passive voice, but it changes the roles of the subject and object from the original sentence. The original sentence is about the wind fanning the fire, not the other way around.

4. The smouldered fire was fanned by the fast wind. (Incorrect) This option changes the tense and meaning of the original sentence. The original sentence talks about the wind fanning the smouldering fire, not a fire that has already been smouldered. The correct form is in the past tense: "The smouldering fire was fanned by the fast wind."