Explanation:

- The northern plain of India is known for its fertile soil and is primarily formed by the deposition of silt by rivers like the Ganges, Yamuna, and their tributaries.

- Alluvial soil:

- This is the correct answer. The northern plain is predominantly made up of alluvial soils, which are deposited by the major rivers.

- They are highly fertile and support extensive agriculture.

- Laterite soil:

- Found in high rainfall areas like the Western Ghats, not typical of the northern plains.

- Less fertile due to leaching.

- Arid soil:

- Found in desert areas like Rajasthan.

- Sandy and lacking in moisture content.

- Black soil:

- Known as Regur, it's found in the Deccan Plateau, suitable for cotton.