Final answer:
The word 'whimsical' best completes the sentence about Cinderella's castle, as it suggests a charming, fanciful or playfully quaint quality.
Step-by-step explanation:
The vocabulary word that best completes the sentence, Cinderella's castle in Walt Disney World, a fanciful place, would be described as, would be whimsical. The castle is nothing short of magical and out of the ordinary, encapsulating an essence of fantasy and playfulness that the word whimsical conveys. Other choices like 'apprehensive', 'candor', or 'slovenly' don't fit the context of a charming and imaginative place like Cinderella's castle.