Sentence:  According to one authority, nearly 40,000 of the Sleswick Danes had become <WSD>'optants</WSD>'—that is, had taken the 'option' of Danish nationality—or had emigrated, by the end of 1880. 
Meaning of the word marked with <WSD> tags:  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.