Degradation is the correct answer.

Degradation refers to the process of wearing down or eroding of the Earth's surface. It involves the removal and transportation of weathered materials such as rocks, soil, and sediment from higher elevations to lower elevations. This phenomenon is particularly common in highland areas where gravity plays a significant role in the erosion process.

Below are the key points explaining the phenomenon of degradation:

Gravity as a Driving Force:
- Gravity is a major driving force behind the erosion of highlands. It causes the movement of rocks and sediments downhill, leading to the gradual wearing down of the highland areas.
- The force of gravity acts on loose materials, causing them to move downslope through various erosion processes.

Weathering and Erosion:
- Weathering is the breakdown of rocks into smaller particles due to exposure to elements such as water, wind, temperature changes, and biological activity.
- Erosion is the transportation and removal of these weathered materials by natural agents such as water, wind, and ice.

Mechanisms of Erosion in Highlands:
- Water Erosion: In highland areas, water plays a significant role in erosion. Rainfall, streams, and rivers carry away weathered materials from higher elevations to lower elevations.
- Wind Erosion: Wind can transport smaller particles such as sand and dust over long distances. In highlands, strong winds can pick up loose materials and carry them away, contributing to erosion.
- Glacial Erosion: In areas with glaciers, the movement of ice can cause significant erosion by plucking and abrasion. As glaciers move downhill, they scrape and carve the landscape, carrying away debris.

Effects of Degradation:
- Degradation of highlands can result in the formation of valleys, gorges, and canyons. Over time, the constant erosion can lead to the creation of dramatic landforms.
- The eroded materials are transported and deposited in lower-lying areas, contributing to the formation of sedimentary deposits such as alluvial plains and deltas.

In conclusion, degradation is the process of erosion that occurs in highland areas. Gravity, in combination with weathering and various erosion mechanisms, plays a vital role in wearing down the highlands and shaping the landscape over time.