Who can initiate an optional referendum at the national level?

Bahrain Constitution

Article 43

The King may conduct a popular referendum on important laws and issues connected with the interests of the State. The issue on which the referendum has been held is considered to have been agreed upon if approved by a majority of those who cast their votes. The result of the referendum shall be binding on all and effective from the date it is declared, and it shall be published in the Official Gazette.