The correct answer is option 4: per square kilometer. Population density is a measure of how many people are living in a specific area. In India, population density is defined as the number of persons per square kilometer. This means that the total population of a given area is divided by the total land area in square kilometers to calculate the population density. It is an important indicator of how crowded or densely populated an area is. It helps in understanding the distribution of people across different regions and can provide insights into resource allocation, urban planning, and infrastructure development. It is important to note that option 1 (per square mile), option 2 (per lakh square mile), and option 3 (per lakh square kilometer) are incorrect and should be disregarded.