The statement that best explains why their stories are unique is B. Aesop's fables were passed from one generation to the next by word of mouth, while Harun al-Rashid's collection was written down in Arabic. This detail highlights a key difference in the way the stories were transmitted and preserved over time. It also suggests that Aesop's fables may have evolved over time as they were passed down orally, while Harun al-Rashid's stories were more likely to have remained consistent over time due to their written form.