A. And every fair from fair sometime declines,
Assonance is the repetition of identical or similar vowel sounds within a line of poetry or prose. In line A, the repetition of the vowel sound "ai" in the words "fair," "from," "fair," and "sometime" is an example of assonance. Line B does not contain any repetition of vowel sounds, and line C contains the repetition of the vowel sound "i" in the words "Shall," "compare," "thee," and "day," but this is not an example of assonance because the vowel sounds are not identical or similar.