Answer:
The correct answer is D) Population shifts as shown by the U.S. Census.
Explanation:
Population shift here refers actually to electoral shifts. Boundaries and districts are remapped based on the polarisation of electorates within specific geographical areas.
In America, this usually takes place after censuses which are held decennially (that is every ten years).
In some cases, this process is intentionally used by politicians to give an advantage to one political party over the other. That is a district can be recreated to have a large population so that its votes can be higher when required.
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