Part A: The imagery in the poem's first stanza affects its mood in the way described in letter A. The woodland paths, the lake that mirrors a still sky and the swans bring the reader a feeling of peace and quiet, pure serenity. The first stanza and its imagery only aim at creating a peaceful and tranquil mood, and nothing else. There isn't (not even in an implicit way) any sadness, darkness, and no romantic mood. It's a mere description of the scenery of the place.
Part B: The statement which best describes how the mood named in Part A changes in the poem's second stanza is the one we see in letter C. The departure of the swans symbolize constancy and beauty in the sense that that's how constant beauty is: it flies away when we least expect, as we get older. It gives the poem a more melancholic mood since the almost straight-forward scenery description we see in the first stanza is now followed by the narrator reflecting upon the passing of time ("The nineteenth Autumn has come upon me/ Since I first made my count) and how beauty (the one described in the first stanza) can be gone right before our eyes, and very quickly ("I saw, before I had well finished/ All suddenly mount/ And scatter wheeling in great broken rings/ Upon their clamorous wings).
