To effectively model mass wasting by water using the activity materials, the most suitable option would be: Put a marble on the top edge of the mountain and pour water down the mountain.
This setup simulates how water can erode materials on a slope, causing them to move downward, which reflects the process of mass wasting. The marble represents debris or soil that can be dislodged and transported by the flowing water, demonstrating the relationship between water and mass wasting.