Final answer:
The water content in the soil is the most influential factor in the downslope movement of weathered particles during mass wasting.
Step-by-step explanation:
The most influential factor in the downslope movement of weathered particles during mass wasting is the water content in the soil. When the soil becomes saturated with water, it becomes heavier and loses its strength, making it more prone to sliding down the slope.
The presence of vegetation can also help stabilize slopes by preventing erosion and providing root reinforcement. Wind speed and altitude are not as influential in the downslope movement of weathered particles.