The correct option is A Metaphor
'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 life of the speaker is compared to that of a happy dream, which cannot be literally true. Thus, the correct answer is Option A) - Metaphor. All the remaining options are also literary devices, but they have not been used in the given sentence.