Final answer:
Mass wasting is most likely to occur during heavy rain, as it decreases sediment cohesiveness and increases material weight, leading to slope instability.
Step-by-step explanation:
Mass wasting is most likely during heavy rain. Mass wasting, or the downslope movement of soil and rock due to gravity, can occur anytime sediments are loosened and hydration is one of the key processes that can promote this instability. Heavy rains lead to the saturation of soil and sediment, which decreases their cohesiveness and increases the risk of a mass wasting event, such as a landslide, mudslide, or debris flow. Extreme moisture from heavy rainfall can also cause an increase in the weight of the slope materials, which can further compromise slope stability and failure.