f
Explanation:
In the lines 417 to 421, the word f is alliterated several times in the verse written in iambic pentameter giving it a rhyming feet and allowing the words with f to stand out as important in the lines.
The words that alliterate are
fens
forth
feathered
fledged
The words do not have to be adjacent to each other spatially but metrically which these words do, albeit not being next to each other. Hence they are in alliteration.
