Gully erosion occurs when running water cuts through clayey soils and creates deep channels. These channels can have significant impacts on the environment and landforms. Here's a detailed explanation of why the correct answer is option 'A' - Gully erosion:

1. Definition of Gully Erosion:
- Gully erosion is a form of soil erosion characterized by the development of deep channels or gullies in the landscape.
- It occurs when water flows quickly over the land, removing soil and creating channels that can become deeper and wider over time.

2. Process of Gully Erosion:
- When rainfall occurs, water infiltrates into the soil. In clayey soils, water absorption is limited due to the high clay content and low permeability.
- As a result, excess water runs off the surface, picking up speed and carrying soil particles with it.
- The force of flowing water cuts into the soil, forming small channels. Over time, these channels deepen and widen, forming gullies.

3. Impact of Gully Erosion:
- Gully erosion has several negative consequences on the environment and human activities.
- Loss of fertile topsoil: Gully erosion removes the top layer of fertile soil, which is essential for agriculture and plant growth.
- Decreased water quality: The eroded soil can be carried into water bodies, leading to sedimentation and decreased water quality.
- Habitat destruction: Gullies can disrupt habitats for plants and animals, causing loss of biodiversity.
- Increased flooding: Deep channels formed by gully erosion can divert water flow, leading to increased flooding in downstream areas.
- Land degradation: Gully erosion contributes to overall land degradation, reducing the productivity and usability of the affected land.

4. Differentiating Gully Erosion from Sheet Erosion:
- Sheet erosion, on the other hand, refers to the uniform removal of a thin layer of topsoil across a large area.
- It occurs when rainfall or irrigation water flows over the land as a thin sheet rather than concentrating in channels.
- Sheet erosion is more common in sandy or loamy soils, where water can easily infiltrate and flow over the surface.

In conclusion, when running water cuts through clayey soils and creates deep channels, it leads to gully erosion. Gully erosion has significant detrimental effects on the environment, including the loss of fertile soil, habitat destruction, and increased flooding. Therefore, the correct answer is option 'A' - Gully erosion.