Who can initiate an optional referendum at the national level?

Sweden
Sweden'sConstitution of 1974 with Amendments through 2012
https://www.constituteproject.org/constitution/Sweden_2012.pdf
Part 15 Motionscalling for a referendum on a matter of fundamental law or for a declaration ofno confidence
Art 15
A motion calling for a referendum ona matter of fundamental law or for a declaration of no confidence shall be putforward at a meeting of the Chamber. The motion shall be delivered in writing assoon as it has been put forward.