The sentence that emphasizes that the subject of the sentence is doing the action is A. The farmer sold over forty pumpkins at the local market on Saturday.
Option A is in active voice, while the others are in passive voice. One of the reasons for choosing to use passive or active voice is the emphasis on the subject and doer of the action. Passive voice is usually used when the subject is not really important and what you want the audience pay attention to is the action itself. That is why, in passive voice, the agent ("by the farmer") can and usually is ommitted.
In active voice, on the other hand, the focus is on the person that is doing the action rather than on the action itself.