The correct answer is (a).
Kerala had the highest density of population in 1981, followed by West Bengal, Bihar, and Madhya Pradesh.
Kerala has a population density of 858 people per square kilometer, which is the highest in India. This is due to a number of factors, including its small size, its long coastline, and its fertile land.
West Bengal has a population density of 809 people per square kilometer. This is due to its large population, its fertile land, and its location in the Ganges River basin.
Bihar has a population density of 804 people per square kilometer. This is due to its large population, its fertile land, and its location in the Ganges River basin.
Madhya Pradesh has a population density of 212 people per square kilometer. This is due to its large size, its relatively low population, and its location in the central part of India.