Final answer:
Rural regions experienced the least significant population shift compared to other areas of the country.
Step-by-step explanation:
The areas of the country that experienced the least significant population shift are rural regions. The data indicates that the population density of cities, especially suburban areas, has grown steadily since the 1950s, leading to a rural-to-urban population shift. Therefore, urban centers, suburban areas, and coastal regions have all experienced more significant population shifts compared to rural regions.