C. It changes the tone from one of downcast complaint to one of gladness and celebration.
Explanation:
The beginning of the poem has a tone of complaint. The speaker tells us that the day was not an enjoyable one. It was the shortest day of the year, the air was dead and the sky was gray. However, the symbolic description of music that is included afterwards changes the tone from one of complaint to one of gladness and celebration. Towards the end of the poem, the poet's words become more positive and he concludes with a very happy statement.
