Answer:
A) Natural selection caused the pigweed to mutate, creating a new triazine-resistant species.
Explanation:
Due to constant use of triazine, the weed will build resistance to it over time unlocking the herbicides codes and making it ineffective.
Natural selection will play out allowing this weeds to bring about a sudden change in their genome that will become resistance, this resistance breed is what will then be produced over time because mutation is heritable and the herbicide then becomes ineffective and the weed continues to multiply and increase in volume.