Most densely populated region of India is-

Option A is correct. The northern plain is richly endowed with alluvial deposits from the rivers originating from the Himalayan mountains, hence it is highly fertile and suitable for farming. The rivers also make possible cheap transport. Thus, the presence of perennial rivers, rich fertile soil and means of cheap transport greatly increases the carrying capacity of the land. Hence, the Central plains is the most densely populated region of India.

Option B is incorrect. The Thar desert is an arid-dry region. The soil is also not as fertile as the plains, and the rivers dry up in the summer. Hence, the land is incapable of holding large population.

Option C is incorrect. The Deccan plateau is rich in mineral content, but the lack of perennial water resources makes it vulnerable to the vagaries of weather. Hence, it cannot support huge densities of population.

Option D is invalid, since there is a correct answer for the question.