The farmer is working . . . . . . . . his field.

Answer: Option C

Option	Example	Explanation
Option A: "at"	The farmer is working at the market.	The preposition "at" is typically used to indicate a specific point or location. In the example, it suggests that the farmer is working in a general location, like the market, rather than being specifically engaged with the field. "At" is not appropriate when referring to someone working directly on the surface or area of the field.
Option B: "in"	The farmer is working in the barn.	The preposition "in" is used to denote something enclosed or inside a specific area or space. For instance, "in the barn" suggests that the farmer is inside a building. "In" might be used if the field were somehow enclosed, but generally, it's not the correct choice for referring to the open area of a field.
Option C: "on"	The farmer is working on his field.	"On" is the correct preposition because it indicates that the farmer is working directly on the surface of the field. The field is a surface or area where the work is being performed, making "on" the most appropriate preposition to use in this context.
Option D: "over"	The farmer is working over the fence.	The preposition "over" suggests a position above something or across from one side to another. In this context, "over" would imply that the farmer is working above the field, which doesn’t make sense in the context of physically working on the land.

Conclusion:
The correct preposition to use is "on" because it accurately conveys that the farmer is engaged in work directly on the surface of the field. The other options either imply a different location or context that does not fit the intended meaning of the sentence.