The correct answer is: a) Large number of people per square kilometer.
Bihar has a population density of 1,106 people per square kilometer, which is the third highest in India. This means that there are a large number of people living in a relatively small area. This can lead to problems such as overcrowding, pollution, and a lack of resources.
Option b) is incorrect because Bihar has a high proportion of urban population. In fact, the urban population of Bihar is 11.3%, which is lower than the national average of 31.1%.
Option c) is incorrect because Bihar has a low birth rate. The birth rate in Bihar is 21.1 per 1,000 people, which is lower than the national average of 22.2 per 1,000 people.
Option d) is incorrect because Bihar has a variety of natural resources, including coal, iron ore, and limestone.