The text in question, as taken from Beowulf, is an example of alliteration.
Alliteration is the repetition of a consonant sound within a line of poetry.
The tracks on the trail were easy to trace.
Here, the consonant "t" sound is repeated in the words "track," "trail," "to" and "trace."
Parallelism is where the components of a phrase, or sentence, are balanced (showing the words have the same importance). An example of parallelism is as follows:
The boys went swimming, running and biking. All of the verbs are balanced given they all end in "ing."
Given that the sentence does not include multiple similar parts of speech, parallelism is not an option.
A metaphor is the comparison between two typically unlike things. An example of a metaphor is as follows:
My life is a roller coaster.
Here, a the speaker's life is compared to a roller coaster (which means that the speaker's life is filled with ups and downs, twists and turns.
Onomatopoeia is when a word is formed using the sound as a suggestion to its spelling. Examples of onomatopoeia are:
bang, boom, moo, meow, splat.
it is definitely alliteration because of the repetition of the consonant letter 't'
It's probably alliteration because "tracks" "trail" and "trace" all start with "t". In parallelism, you would need two or more words, phrases, or clauses that are similar in length and grammatical form. If it was a metaphor, the "tracks" or the "trail" would have been symbolic of something else in the play. Finally, if it were onomatopoeia, there would have to be the use of words (such as hiss or murmur) that imitate the sounds associated with the objects or actions they refer to. None of the words in the quote does that, therefore, the phrase is an example of alliteration.
~I hope this helps! :)