Who can initiate an optional referendum at the national level?

Antigua and Barbuda
The Antigua andBarbuda Constitution Order 1981
http://aceproject.org/ero-en/regions/americas/AG/Antigua%20and%20Barbuda%20-%20Constitution
The Referendum Act1997
http://aceproject.org/ero-en/regions/americas/AG/antigua-and-barbuda-the-referendum-act
The Referendum Act 1997