An alien who has lived in Nigeria for twenty years may acquire citizenship by

Acquiring citizenship through naturalization is a legal process that involves fulfilling specific requirements, such as living in a country for a certain number of years, having a clean criminal record, and passing language and/or citizenship tests. In Nigeria, an alien who has lived in the country for twenty years and has shown evidence of a clear intention to continue residing in the country may apply for citizenship through naturalization. This process involves applying to the President through the Minister of Interior, and if the application is successful, the alien will be granted Nigerian citizenship and issued a certificate of naturalization. Therefore, the answer to the question is "naturalization".