'Metaphor' is a literary device / figure of speech in which a word or a phrase is given / applied to an action, object or event where it is not literally applicable. In simpler terms, a metaphor is a comparison that is drawn between two completely unrelated things. In this case, the sound of the subject's laughter is compared to music due to how pleasant it sounds, which cannot be literally true. Thus, the correct answer is Option C) - Metaphor. All the remaining options are also literary devices, but they have not been used in the given sentence.