Final answer:
The line from the narrative "I'm not thirteen yet" that best supports the main lesson is likely "I'm not thirteen yet," as it echoes the title and suggests a theme of understanding one's current age and stage in life.
Step-by-step explanation:
The lines that best support the main lesson of the narrative "I'm not thirteen yet" need to be considered based on the context of the story. Since we don't have the full narrative here, we can analyze the options provided.
The line "I'm not thirteen yet." directly references the title of the narrative and likely encapsulates the central message about the anticipation of growing up and perhaps the realization that the character isn't quite ready for it. This line seems to emphasize an understanding or acceptance of one's current stage in life, which could very well be the core lesson of the story.
Other options like "I can't wait to turn thirteen." or "Being twelve is so boring." express a desire for change or a feeling of ennui, but they don't necessarily reflect a lesson or moral. While "I'm excited for my birthday." conveys enthusiasm, it doesn't specifically tie into the thematic age-related wisdom that the title suggests.