The cause-and-effect diagram that best illustrates one main effect of informed citizens on the political system is:
Citizens make informed voting decisions.
Explanation:
1. When citizens are well-informed about political issues, candidates, and policies, they are better equipped to make informed voting decisions during elections.
2. Informed voting decisions lead to the selection of candidates who align with the values and priorities of the electorate.
3. This process promotes a more representative and effective political system where elected officials reflect the will of the informed citizenry.
Therefore, out of the given options, "Citizens make informed voting decisions" is the most direct and impactful effect of informed citizens on the political system.