Who can initiate an optional referendum at the national level?

Constitution of Nepal

(adopted in 2007)

http://www.wipo.int/wipolex/en/text.jsp?file_id=189180

Article 157(1).

Save as otherwise provided elsewhere in thisConstitution, if a two-thirds majority of the total number of the then membersof the Constituent Assembly decides that it is necessary to decide any issue ofnational importance through referendum, such issue may be decided throughreferendum