Alluvial soils are rich in:

The correct answer is: A) Nutrients.

Alluvial soils are formed by the deposition of sediments from rivers and streams. These sediments are rich in nutrients, which makes alluvial soils ideal for agriculture. Alluvial soils are also typically well-drained and have a good texture, which makes them easy to work with.

Option B, moisture, is also a characteristic of alluvial soils. However, it is not the most important characteristic. Alluvial soils are also typically sandy, but this is not a desirable characteristic for agriculture. Rock fragments are not a characteristic of alluvial soils.