The dialogue option that best helps develop both characters for readers is:
C. "No way!" Sadie beamed. "Clear your schedule. We're going to have a blast."
"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 option effectively highlights Sadie's optimism and excitement at the prospect of the concert, showcasing her tendency to focus on the positive. In contrast, Marco's skeptical response underscores his brooding nature and reluctance to believe in good fortune. The exchange illustrates their differing outlooks on life and sets up an interesting dynamic between the two friends.