Final answer:
Clayey soils indeed facilitate mass movement because their fine, cohesive particles become slippery when wet, reducing friction and helping material to slide down slopes.
Step-by-step explanation:
The statement that clayey soils or clayey subsoils greatly facilitate mass movement is True. Clayey soils have fine particles that, when moistened, can become very slippery. This property can lead to easier movement of material down slopes, which is a process known as mass movement or mass wasting. Clay's cohesive nature when wet contributes to a reduced frictional resistance against movement. Moreover, when water is added to clayey soils, the material becomes even more plastic and cohesive, which assists in the process of mass movement, especially during significant rainfall or other events that saturate the soil with water.