Most rapid and spectacular mass movement events occur . a. in rugged, geologically young mountains b. in relatively old, heavily eroded mountains c. along the flanks of extinct volcanoes d. along the banks of meandering streams

in areas of rugged, geologically young mountains

Explanation:

Most rapid and spectacular mass movement events occur in rugged, geologically young mountains due to slopes change over time where mass wasting and erosional processes slowly change these  rugged,geologically young mountains to more subdued terrain

a. in rugged, geologically young mountains

Explanation:

Mass wasting occurs on a slope. Mass wasting also known as mass movement is the mass movement of soil, rocks, regolith, debris , mud downward along a slope.

Most rapid mass movement event will definitely occur in  rugged, geologically young mountains. The slope of this young mountain are still undergoing geologic event like mass wasting . The event are shaping the mountain to become stable in the future.

Generally, as the terrain ages their is less occurrence of mass wasting. The older terrain would have gained more stability than a young slope which is still shaping rapidly and constantly.