Who can initiate an optional referendum at the national level?

Referendum Act

Proclamation of referendum:

3. (1) Where the Governor in Council considers that it is in the public interest to obtain by means of a referendum the opinion of electors on any question relating to the Constitution of Canada, the Governor in Council may, by proclamation, direct that the opinion of electors be obtained by putting the question to the electors of Canada or of one or more provinces specified in the proclamation at a referendum called for that purpose.