Which of the following questions can be answered scientifically? 1) Should we just let the river flow where it wants? 2) What can be done to stop erosion? 3) Is erosion control worth the financial costs? 4) Can planting cover crops reduce erosion?

4 is a good answer. The question ‘can planting cover crops reduce erosion’ can be easily answered by creating a scientific study. You could create an experimental group where cover crops are planted and a control group where they are not, and measure the erosion of each group.