optant

plural optants

1. A person who lives in a region undergoing a change of sovereignty and thus may choose between retaining their old citizenship or opting for the citizenship of the new sovereignty.First known use 1914

   According to one authority, nearly 40,000 of the Sleswick Danes had become 'optants'—that is, had taken the 'option' of Danish nationality—or had emigrated, by the end of 1880.

   1914, W. R. Prior, Oxford Pamphlets: North Sleswick under Prussian Rule, London: Oxford University Press, page 9

2. A person who opts into, out of, or for something.First known use 1943