The correct option is A NO CHANGE
A conjunction that represents a choice between the alternatives in a sentence, is called a disjunctive or alternative conjunction. Some of these are in pairs "neither...nor", "either...or" and so on. In the given statement, the underlined word "nor" is paired with "neither", which is correct because it represents the alternatives "planes flying" and "trains running" in a negative sense. Hence no change is necessary, option A is correct. 'Or' is paired up with 'either' and not 'neither', 'either' isn't in the sentence, hence it can't be used in the given statement, option B is incorrect. 'And' is a cumulative conjunction that merely connects or adds one sentence/clause to another. It's not an alternative conjunction, hence option C is incorrect. 'Yet' is an adversative conjunction used when two clauses in a sentence are contrast in their meaning. Since the sentence has alternatives, and 'yet' is not an alternative conjunction, option D is incorrect.