Final answer:
An instance of persuasive speaking is when a volunteer convinces a group of citizens to sign a petition, as it involves direct persuasion through reasoning or argument. The correct option is C. A volunteer convincing a group of citizens to sign a petition.
Step-by-step explanation:
Among the provided a volunteer convincing a group of citizens to sign a petition is an instance of persuasive speaking. This is because the act of persuading involves trying to convince someone to do something or believe in something through reasoning or argument, which is exactly what a volunteer does when trying to garner signatures for a petition. While the other options may involve speaking, they do not necessarily involve persuasion in the same direct way that convincing someone to sign a petition does.