Which is the best example of a direct democracy action?

A. Voters select the new Senator to replace the one that is retiring
B. Voters select a slate of city council members
C. Voters attend a session at the State Capitol to lobby their representatives
D. Voters initiate a petition that keeps taxes from rising

D. Voters initiate a petition that keeps taxes from rising is the best example of a direct democracy action, as it involves citizens directly proposing and enacting a policy change through their own initiative and action. The other options involve voting or lobbying representatives, which are indirect forms of democracy.