Final answer:
The best example of erosion among the given choices is rainwater washing dirt down a hill, as it demonstrates the movement and transport of soil by water, which is a key component of erosion. The correct option is c.
Step-by-step explanation:
The best example of erosion is C) rainwater washing dirt down a hill. Erosion is the process by which natural forces move weathered rock and soil from one place to another. When rainwater flows down a hill, it picks up loose soil along the slope and transports it to another location. This movement of soil exemplifies erosion, particularly when the vegetative cover is reduced, making the soil more susceptible to being washed away.
Contrastingly, sand being deposited on a beach is a result of deposition, wind blowing trees over is a form of disturbance, and tree roots growing between rocks is more related to weathering as they physically break rocks apart. Clastic sedimentary rocks demonstrate erosion through their formation when particles are carried through water or air and then settle out.