The Poet uses litotes as a literary technique in the sentence That [sword] was not useless to the warrior now from Beowulf.
Litotes is a technique that communicates in a positive message through a negative structure, we can see in the sentence the use of was not, but the general meaning is a positive one saying that the sword was in fact really useful.
The other options are not correct because those techniques make reference to sounds and metrics or use of a combination of words no the general meaning of a sentence.
