Answer: A) Alliteration.
Explanation: alliteration is a literary device that consists in the repetition of the beginning sounds of consecutive words, or words that are close to each other. In the given sentence we can see a clear example of alliteration because of the repetition of the "f" sound at the beginning of the words "foxes," "fled," "foes," "forests," and "fens," so the correct answer is the corresponding to option A.