"So sober sometimes serious Sam smiles on silly things" is a /an _______.

A Hyperbole refers to exaggeration

Assonance refers to the resemblance of sound between syllables of nearby words like cold, culled etc. 

Anaphora is the repetition of a word or phrase at the beginning of successive phrases clauses or sentences. 

Alliteration refers to the repetition of the same letter at the beginning of consecutive words. 

Since 's' is repeated in the beginning of words here, the answer is (d) alliteration.