Answer:
[D] Heavy use of pesticides.
Step-by-step explanation:
Conserving water, multicropping, and responsible irrigation are approaches to sustainable crop production. (Sustainable: Able to be maintained at a certain rate or level).
Heavy use of pesticides is not an approach to sustainable crop production. This might kill parts of the crop as well as make the food unhealthy to eat. It would be "unmaintainable".