Final answer:
The United States Constitution requires a national census every ten years to ensure fair representation in Congress by determining the population of each state.
Step-by-step explanation:
The United States Constitution requires a national census to be taken every ten years in order to ensure fair representation in Congress. The census counts the number of people living in the United States and determines the population of each state. Based on this population data, Congress determines how many representatives each state will have. This process helps to distribute representatives proportionally and avoid over or underrepresentation.