The correct answer is option 'B', external process.

External processes are the continuous wearing down and rebuilding of the land surface. These processes are driven by external forces such as weathering, erosion, and deposition.

There are several key points to understand about external processes:

1. Definition of external processes:
- External processes refer to the physical and chemical processes that occur on the Earth's surface.
- These processes are primarily driven by external forces, such as weathering, erosion, and deposition.
- External processes are responsible for shaping the Earth's surface over long periods of time.

2. Weathering:
- Weathering is the process by which rocks and minerals are broken down into smaller pieces.
- It can occur through physical processes like freeze-thaw cycles, abrasion, and exfoliation.
- Chemical weathering also takes place, involving chemical reactions that alter the composition of rocks and minerals.

3. Erosion:
- Erosion involves the removal and transportation of weathered materials by various agents such as water, wind, ice, and gravity.
- Water erosion occurs through the action of rivers, streams, and rainfall.
- Wind erosion is common in arid and desert regions, where strong winds can pick up and transport sediment.
- Glacial erosion occurs when glaciers move and carve out valleys and other landforms.
- Gravity plays a role in mass wasting, which refers to the downslope movement of material due to gravity.

4. Deposition:
- Deposition occurs when eroded materials are dropped or settled in new locations.
- It is often associated with the slowing down or cessation of erosional processes.
- Deposition can result in the formation of various landforms, such as deltas, beaches, and alluvial plains.

Overall, external processes are responsible for the continuous wearing down and rebuilding of the land surface over time. They play a crucial role in shaping the Earth's surface features and are an important part of the geological cycle.