The southern parts of Bihar have a mix of alluvial and red soils, the latter being less fertile due to:

The correct answer is: d) Leaching of nutrients.

Leaching is the process by which water moves through soil and carries away dissolved minerals and other nutrients. This can happen naturally, but it is often accelerated by human activities such as agriculture and deforestation.

In the southern parts of Bihar, the red soils are less fertile because they are more prone to leaching. This is because the red soils are sandy and have a low clay content. Clay particles help to hold onto nutrients, so when they are leached away, the soil becomes less fertile.

The other options are not correct because:

• High salinity is not a major problem in the southern parts of Bihar.
• Excessive waterlogging is not a major problem in the southern parts of Bihar.
• The presence of rocks and pebbles does not make soil less fertile. In fact, rocks and pebbles can help to improve drainage and aeration in the soil.