Who can initiate an optional referendum at the national level?

Constitution 2007

https://www.constituteproject.org/constitution/Montenegro_2007.pdf

Article 82: Responsibility

The Parliament shall:

[…]

11. Call for the national referendum;

Article 93: Proposing laws and other acts

The right to propose laws and other acts shall begranted to the Government and the Member of the Parliament.

The right to propose laws shall also be granted to sixthousand voters, through the Member of the Parliament they authorized.

The proposal to call for the national referendum maybe submitted by: at least 25 Members of the Parliament, the President ofMontenegro, the Government or at least 10% of the citizens with the right tovote

Law on Referendum (2001)

http://www.legislationline.org/documents/id/3934

Article 5

The decision on calling a republican referendum shall be made by theAssembly of the Republic, by majority of votes of the total number ofrepresentatives in the Assembly.

The decision on calling a referendum shall determine specifically, thewording of the question on which the citizens are to pronounce themselves inthe referendum, as well as the date of holding the referendum.

The decision to call a referendum shall be publicized in the mannerapplicable to the regulations of the authority calling the referendum.