The farmer is working . . . . . . . . his field. A. at B. in C. on D. over

The correct answer is C. on.

The farmer is working on his field because he is physically located on the field. He is not working at his field, which would imply that he is working in an office or other building that is located near the field. He is also not working in his field, which would imply that he is working underground or inside a structure that is located on the field. Finally, he is not working over his field, which would imply that he is working in the air above the field.

The other options are incorrect because they do not accurately describe the location of the farmer.