What is one main reason for high population in the northern plains of India?

Correct answer: Fertile alluvial soil

The direct answer is B: fertile alluvial soil. Population is often high where people can grow enough food and earn a living. The northern plains, especially the Ganga plain, receive fine soil deposited by rivers. This alluvial soil is generally deep and suitable for crops such as rice, wheat, sugarcane and pulses. Step by step: fertile soil supports farming; farming provides food and work; food and work attract people; therefore settlements become dense. Option A is wrong because very large glaciers make land cold and difficult for farming, and glaciers are not the main feature of the northern plains. Option B is correct because fertile alluvial soil supports intensive agriculture and settlements. Option C is wrong because the northern plains are not always dry; many parts receive river water and seasonal rainfall. Option D is wrong because the plains have vegetation, cultivated fields and many settlements. Remember: fertile plains usually mean productive agriculture and dense population.