Answer is: voting in elections. Whereas paying taxes, going to school, and obeying laws also can be better described as duties. We need to understand that citizen's also are responsible for influencing the current state of things in politics, so voting in elections is in the most cases the most representative instance for all citizen's to gather and discuss the future of the elections to be held. They also become responsible in the organizationg of the general elections and supporting the candidates before they become even nominated. As a political process not only the political class becomes involved so we can think of voting as a main responsability of the citizens.