The term 'Doubtful voter' or 'D-voter' has been primarily used in which northeastern state?

Assam has identified around 1.2 lakh people as ‘D’ (Dubious or Doubtful) voters, with 41,583 declared as foreigners. The D-Voter concept is specific to Assam, where migration and citizenship are major political issues. It was introduced in Assam in 1997. D-Voters are those who cannot prove their Indian nationality. People with questionable or disputed citizenship were marked as D-Voters during the National Register of Citizens (NRC) preparation. The term ‘Doubtful voter’ is not defined in the Citizenship Act of 1955 or the Citizenship Rules of 2003. The Citizenship Rules of 2003 outline the process for creating the National Population Register and National Register of Indian Citizens.