What action is considered a formal expression of opinion on public matters?

Voting is considered a formal expression of opinion on public matters as it is a structured and official process through which citizens participate in the democratic system. By casting a vote, individuals express their preferences and opinions regarding policies, candidates, and various issues that affect their lives and communities. This process is institutionalized, often taking place in a designated location and following established rules, which reinforces the legitimacy of the public's opinion in the governance process.

In contrast, a plebiscite is a specific type of voting usually focused on a particular issue, while protests and campaigning involve raising awareness and mobilizing support around issues rather than formally expressing individual opinions through a recognized electoral process. While protests and campaigning play important roles in influencing public opinion and democratic engagement, they do not carry the same formal weight in terms of decision-making as the act of voting does.