Can a 501c3 NPO have a 401(k) plan alone, or do they have to have a 403(b)?

Answer- both or either but employee deferrals are limited to aggregate of 15/20k under both plans (but employees of some c3 can defer an additional 3k under a 403b plan for 5 yrs). There are separate 415 limits for 403b and 401k plans. C3 can also adopt a 457 plan to defer 15k for members of the top hat group in additon to contributons under 401k/403b plans.