Explanation:

- Population Density: This measures the number of people living per unit area. It's a way to understand how crowded a place is. Correct Answer

- Population Sex Ratio: This indicates the proportion of males to females in a population. It's used to study gender distribution.

- Population Movements: This refers to the migration of people from one place to another. It doesn't measure the number per unit area.

- Population Growth Rate: This measures the increase in the number of people in a population over time. It's about growth, not density.