Contour bunding is a method of soil conservation that is not specifically used in any of the options provided in the question. Let's discuss each option and why it is not the correct answer:

a) Desert margins, liable to strong wind action: While contour bunding can be effective in preventing soil erosion caused by wind, it is not specifically used in desert margins. In desert areas, other methods such as sand dune stabilization or windbreaks are more commonly used.

b) Low flat plains, close to stream courses, liable to flooding: Contour bunding is not specifically used in low flat plains close to stream courses. In such areas, other methods like levees or floodplain zoning are commonly employed to manage flooding.

c) Scrublands, liable to spread of weed growth: Contour bunding can be used in scrublands to prevent soil erosion, but it is not specifically used to control weed growth. Other methods such as herbicides or manual weed removal are typically used to control weed growth in scrublands.

Therefore, the correct answer is option D - None of the above, as contour bunding is a method of soil conservation that can be used in various landscapes to prevent erosion and retain moisture in the soil.

Contour bunding involves the construction of small ridges or bunds along the contour lines of a slope. These bunds are typically made using soil, stones, or other locally available materials. The bunds are constructed perpendicular to the slope and are designed to slow down the flow of water, allowing it to infiltrate into the soil rather than running off and causing erosion.

Some key points about contour bunding include:

1. Soil erosion prevention: By slowing down the flow of water, contour bunds help to prevent soil erosion by reducing the force of the water and allowing it to infiltrate into the soil.

2. Moisture retention: The bunds also help to retain moisture in the soil by trapping water behind them. This can be particularly beneficial in areas with limited rainfall or during dry periods.

3. Nutrient conservation: Contour bunds can help to prevent the loss of nutrients from the soil by reducing the amount of water runoff. This allows the nutrients to remain in the soil and be used by plants.

4. Terracing effect: In addition to their soil conservation benefits, contour bunds can also create a terracing effect on slopes. This can make it easier to cultivate crops or plant trees, as it reduces the slope gradient and provides flat areas for planting.

In conclusion, contour bunding is a method of soil conservation that can be used in various landscapes to prevent erosion and retain moisture in the soil. It is not specifically limited to desert margins, low flat plains close to stream courses, or scrublands prone to weed growth.