Alluvial soils are generally considered:

The correct answer is: a) Highly fertile.

Alluvial soils are formed by the deposition of sediments from rivers and streams. These sediments are rich in nutrients, making alluvial soils highly fertile. Alluvial soils are also well-drained and have good aeration, which makes them ideal for growing a variety of crops.

Option b) is incorrect because alluvial soils are not moderately fertile. They are highly fertile.

Option c) is incorrect because alluvial soils are not of low fertility. They are highly fertile.

Option d) is incorrect because alluvial soils are suitable for agriculture. They are highly fertile and well-drained.