Can someone please tell me which is most appropriate?
(a) "At this time, none of the parties is planning to file a motion"; or
(b) "At this time, none of the parties are planning to file a motion."
Can someone please tell me which is most appropriate?
(a) "At this time, none of the parties is planning to file a motion"; or
(b) "At this time, none of the parties are planning to file a motion."
Either is correct, although I was taught that "None is" is more appropriate in legal proceedings, where every word matters. After subsequently reading Jon Hanna's excellent analysis in answer to a similar question on EL&U, I can only conclude that what I was taught is not correct. I encourage you to read his answer, which concludes that both uses are equally appropriate in any setting, with a few exceptions as noted. See: "None" as plural indefinite pronoun for his answer.
None "is." That's the only correct answer. This is a common error. None means "not one," clearly singular. Read it as "Not one is planning to file.." and it becomes obvious.
None stands for "not one"; thus the only correct way to use it here would be none is. As attested by this quote from Lord Byron's Don Juan:
In Seville was he born, a pleasant city,
Famous for oranges and women—he
Who has not seen it will be much to pity,
So says the proverb—and I quite agree;
Of all the Spanish towns is none more pretty,
Cadiz perhaps—but that you soon may see;
Don Juan's parents lived beside the river,
A noble stream, and call'd the Guadalquivir.