To accurately determine why the poet repeats the word "up" in line 10, we would need to consider the context of the poem and the overall theme and mood conveyed in that particular line. However, based on the options provided, option C seems to be the most compelling choice.
Repeating "up" can indeed serve to emphasize positivity and the idea of looking toward joy and confidence. It suggests an uplifting perspective and encourages readers to adopt an optimistic outlook. Therefore, C. The poet is emphasizing how joy creates confidence by reminding readers to always look up would be a logical interpretation of the repetition.