The odd item from the options is Ballad. A simile is a figure of speech that compares two things using "like" or "as," while a metaphor is a figure of speech that compares two things by saying one thing is another thing. A synecdoche is a figure of speech in which a part of something is used to refer to the whole thing.
A ballad, on the other hand, is a type of narrative poem or song that tells a story in a simple and direct way. While the other options are all figures of speech that are used in writing and speech to make comparisons or convey meaning in a more creative way, a ballad is a specific form of poetry or song. Therefore, ballad is the odd item in this group.