Citizenship can be acquired by:
- Naturalization.
Therefore, the correct answer is: naturalization.
For reference, here are the definitions of the other terms mentioned:
- Nationalism: a sentiment based on common cultural, historical, or political factors that unites a group of people as a distinct nation with a shared identity.
- Indigenization: the process of bringing individuals or groups under the authority or domination of an indigenous group or culture.
- Communalism: a social and political system that emphasizes the importance of community and the common good over individual interests.