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. The given sentence is a positive one, that explains that one of the two people should have arrived as they left long back. So ‘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 ‘Jack or Justin' is in singular ‘either’ is the apt answer, Thus option A is correct while option B is incorrect.