Answer:
C. a chemical treatment.
Step-by-step explanation:
In this case we would be talking about a chemical treatment, since the application of an artificial compound is made, and it is not clarified that it was "ecofriendly".
On the other hand, the question clarifies that the farmer is suffering from the plague. This indicates that he is not preventing it (Integrated pest management), he is treating it.
Finally, it should not be considered as an agronomic treatment, since no real modifications are being made to the way the field is managed.