Issues in relation to which referendums are optional at the national level

Morocco
Answer
Constitutional amendments
Source
Morocco’sConstitution of 2011
https://www.constituteproject.org/constitution/Morocco_2011.pdf
Article 174
The bills and proposals of revision of theConstitution are submitted by Dahir to referendum.
The revision of the Constitution is definitive afterhaving been adopted by way [vole] of referendum.