Answer:
The answer is actually C, Political machines are at the center of urban politics.
Explanation:
Richard "Boss" Croker was a politician, but he also was part of Tammany Hall and that was the largest Political Machine for the Democratic Party. This is where he got the "Boss" nickname, he was a Political Boss. The people revolving around Croker indicate that everything revolved around these Political Machines, everything revolved around Croker.