The requirements of the federal law that spells out the criteria for fair representative districts specifies that each must contain an equal number of what?

Citizens and mandates a census to determine the number of citizens and apportion seats to each state.

Explanation:

As it is been practiced in decades, this is a process of drawing boundaries for electoral and political districts in the US and is usually done every ten years after the census. The US constitution requires each Representative in Congress represent an equal number of citizens and mandates a census to determine the number of citizens and apportion seats to each state.

Redistricting procedures vary by state individual states have the responsibility of drawing legislative boundaries for congressional districts as well as for state legislative districts. In most states, the state legislature districts seats, usually subject to concurrence by the state governor. A few conduct redistricting through an independent or bipartisan commission in order to minimize the impact of partisan or legislative politics. A handful allow an independent commission to propose redistricting plans subject to approval by the state legislature.