B. environment of evolutionary adaptedness.
The answer is capitalist economy.
Based on the socio-political system, in addition to gerrymandering, a significant reason why so many members of Congress are elected from "safe" districts is "<u>the natural geographic clustering of people who share a political ideology."</u>
The demographic analysis has shown that people of the same political ideology tend to stay together in the same geographical area.
One of the reasons is that people living in the same geographical area share similar issues, such as social infrastructures.
Another reason is that people can easily influence one another when they stay in the same locality, increasing the clustering of people who share a political ideology.
Hence, in this case, it is concluded that the correct answer is "<u>the natural geographic clustering of people who share a political ideology."</u>
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