What did she insist on?

The correct answer is that she insisted on seeing the manager at once. This choice conveys a strong sense of urgency and necessity in her request. By using the word "insist," it implies that she is making a firm demand or request that holds significant importance to her. In this context, "seeing the manager at once" suggests that her issue or concern requires immediate attention, emphasizing her determination.

The other options do not capture the same level of urgency or specific demand. For example, having a meeting later or waiting for someone else indicates a more passive or scheduled approach rather than a direct and immediate need. Changing her flight, while it might involve an important decision, does not inherently carry the same strong directive force or immediacy as insisting to see someone right away. Thus, option B clearly aligns with the connotation of "insist," highlighting her assertiveness in wanting to address her issue without delay.