The correct option is A fewer costumes
'Fewer' is used for plural countable nouns, 'less' is used for uncountable nouns. Costume is a countable noun, which is why we can't leave the statement as it is, option A is incorrect. We use 'fewer costumes' which is the correct option (option B). Since less isn't used for non-countable nouns, Option C is incorrect. Option D 'fewer' does not say which noun is fewer, hence the last option is also incorrect due to ambiguity.