Who can initiate an optional referendum at the national level?

Denmark, Constitution 1953, accessed 24 September 2019

Section 42(1). Where a Bill has been passed by the Folketing, one-third of the Members of the Folketing may within three week-days from the final passing of the Bill request of the President that the Bill be subjected to a Referendum. Such request shall be made in writing and signed by the Members making the request. [...]

See also: The Danish Parliament, FACTS AND FIGURES: REFERENDA 2012, accessed 24 September 2019