increasing the slope of the hill spraying herbicides on the hill adding plants to the hill removing grass from the hill
I gave my students this multiple choice question: Many houses are built on the sides of hills. Which of these actions would best prevent houses from sliding down hills after heavy rainfalls?

Only one answer is correct among these 4 choices:
- increasing the slope of the hill
- spraying herbicides on the hill
- adding plants to the hill
- removing grass from the hill

Could you tell me which one is correct?

adding plants to the hill