Final answer:
A referendum (option c) is the best example of direct democracy in practice in the U.S., where citizens directly vote on legislation or government decisions.
Step-by-step explanation:
The best example of direct democracy in practice in the United States is option c. referendum. This is because a referendum involves citizens voting to confirm or repeal decisions made by the government directly. In the context of U.S. democracy, referendums, along with initiatives and recalls, permit citizen participation in creating, affirming, or rescinding legislation, thereby exercising a form of governance where the people act as legislators themselves.
Gerrymandering, on the other hand, is a practice within representative democracy where legislative districts are drawn in a manner that advantages certain political groups, and does not represent an example of direct democracy. Retrospective voting refers to voters making their choices based on the past performance of candidates or parties, which is still within the realm of representative democracy. Issue advocacy involves supporting or opposing particular causes or legislation, but it is not a mechanism for direct legislation by citizens.