The dialogue that best helps develop both characters for readers is:
B. "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 clearly highlights Sadie's optimistic personality as she enthusiastically anticipates a fun experience, while Marco's skeptical nature is emphasized by his hesitation and doubt about getting tickets. Their contrasting viewpoints are well illustrated, providing insights into their characters and the dynamics of their friendship.