The best example of alliteration among the given options is:
B. Fish swim fast in fresh water.
This line demonstrates alliteration with the repeated "f" sound in "fish," "swim," and "fresh," creating a pleasing and rhythmic effect. Alliteration is a literary device where words in a sentence or phrase begin with the same consonant sound, enhancing the flow and musicality of the writing. In this case, the repeated "f" sound emphasizes the speed and agility of the fish swimming in the fresh water, making it a strong example of alliteration.
