To effectively develop both characters, the dialogue that showcases Sadie's optimism and Marco's skepticism while also illustrating their dynamic would be:
B. "I don't know," Marco said slowly. "Are you sure we'll even get tickets? This is going to be a pretty big deal."
This line highlights Marco's cautious nature and skepticism about the situation, showing his tendency to doubt good fortune. In contrast, it sets up Sadie's hopeful attitude, allowing readers to see how she might respond to his doubt. It builds tension between their perspectives, emphasizing their contrasting outlooks and enriching their character development.