The correct passive form of the given sentence is The fields are being ploughed by the farmer.

The active voice of the sentence is The farmer is ploughing the fields. This sentence is in the present progressive tense, which is used to describe an action that is happening now. The passive voice of the sentence is formed by putting the subject of the active sentence after the verb "to be" and the present participle of the main verb. In this case, the subject of the active sentence is "the farmer" and the main verb is "plough." The present participle of "plough" is "ploughing." So, the passive form of the sentence is The fields are being ploughed by the farmer.

The other options are incorrect because:

• The fields are ploughed by the farmer. is in the simple present tense, while the original sentence is in the present progressive tense.
• The fields were ploughed by the farmer. is in the past tense, while the original sentence is in the present progressive tense.
• The fields have been ploughed by the farmer. is in the present perfect tense, which is used to describe an action that has happened in the past but is still relevant to the present. The action of ploughing the fields is not something that is still relevant to the present.