What significant change regarding public opinion began in Ancient Greece?

The significant change regarding public opinion that began in Ancient Greece was the freedom to discuss and debate publicly. This period marked a pivotal moment in the development of democracy, where citizens were encouraged to participate in political discourse and share their opinions in a public forum. The practice of engaging in discussions in spaces such as the Agora allowed for the exchange of ideas and fostered a culture of debate that was essential for democratic governance.

This development laid the groundwork for the evolution of public relations as a practice, highlighting the importance of understanding and influencing public opinion through open dialogue. It emphasized the role of rhetoric and persuasive communication, foundational elements of public relations that remain relevant today. This environment nurtured the idea that public sentiment could be shaped and was vital for the functioning of a democratic society, contrasting greatly with other eras or forms of governance that restricted public expression.

In the context of the other options, the establishment of a monarchy, the advent of social media, and the first newspaper publication do not align with the defining feature of Ancient Greece's contribution to public discourse. Monarchy represents a different political structure, social media was a phenomenon much later in history, and newspapers developed as a medium of communication long after the democratic discussions of Ancient Greece had already evolved.