Final answer:
Political influence in the 21st-century U.S. is characterized by increased polarization, significant impact of social media on politics, and the influence of wealthy donors post-Citizen's United ruling.
Step-by-step explanation:
The statement that best reflects the influence on politics in the United States in the 21st century involves recognizing the polarization of political beliefs, the impact of social media on political engagement, and the influence of economic power through campaign financing. Polarization has been observed through shifting beliefs post-9/11, and partisanship has become more prevalent, with citizens identifying more strongly as conservative or liberal. Social media has substantially changed the political landscape by increasing political participation and the dissemination of information, though it also presents challenges associated with misinformation. Additionally, the 2012 Supreme Court decision in Citizen's United vs. FEC highlighted concerns about the influence of wealthy donors and corporations on American politics, potentially leading to an oligarchy where financial power holds significant sway over political outcomes.