The last / right option - "Snow falls off roofs with slopes"
Explanation:
I do not have the reading, but this answer makes the most sense. Since there is a slope, the snow can fall down because the force of gravity is pulling on it, and since it is not laying on a plane / flat roof, it can fall off.
Have a nice day!
I hope this is what you are looking for, but if not - comment! I will edit and update my answer accordingly. (ノ^∇^)
