Answer:
A fox is seen in the field every week by the farmer.
Step-by-step explanation:
Passive voice is a type of speech type that the subject of the sentence is the receiver of the action. In Active voice, the subject is the giver/performer of the action.
Therefore, from the given sentence which is in active voice, "The farmer sees a fox in the field every week", the subject is "the farmer" and it is the performer of the action.
So, to change it to passive voice, it now becomes, "A fox is seen in the field every week by the farmer."