B. That costume is fancier than mine.
This option is accurate since the comparative form of the word fancy is fancier.
The comparative form of a word is used when comparing two different things. The following sentence contains an example of a comparative word form:
"The sky is bluer today that it was yesterday". In this sentence, the word "bluer" is used since it is comparing two different things- the sky on one particular day versus the sky on another day.