Read the sentence out loud first: "So sober sometimes serious Sam smiles on silly things." Notice how almost every word starts with the same "s" sound.
Now match that pattern to definitions instead of the words themselves. Hyperbole needs an exaggerated claim, which is absent here. Assonance needs matching vowel sounds inside words, but the match here is at the start of words. Anaphora needs the same word or phrase repeated at the start of multiple sentences or clauses, but this is a single sentence, not several.
What is left is the repetition of an initial consonant sound across neighboring words, which is the textbook definition of alliteration. So the answer is D, alliteration.