Either, either of, neither and neither of are distributive pronouns. They are used when there is comparison or choice between two things. ‘ ‘Either’ and ‘either of’ are used to convey a positive meaning, i.e. one of the two options is chosen. While ‘neither’ and ‘neither of’ convey a negative meaning because both are omitted/rejected. Since the given sentence is a positive one, that explains that both of the dresses are good, ‘neither’ and ‘neither of’ cannot be used. Thus options C and D are incorrect. Further ‘either’ is used for singular nouns and ‘either of’ is used for plural nouns. Since ‘dress’ is singular ‘either’ is the apt answer, Thus option A is correct while option B is incorrect.