Concept:An alien is a person who is not a citizen of the country.
A qualified alien may acquire Nigerian citizenship through the legal process of registration.
Explanation:Citizenship by birth is for people born inside the country or to citizen parents, so it does not apply to an alien.
Conferment and internalization are not the standard processes for an ordinary alien to become a citizen.
Citizenship by registration is the correct process for certain aliens.
For example, a foreign woman who marries a Nigerian citizen can acquire citizenship by registration.
Also, an adult born outside Nigeria whose grandparent is a Nigerian citizen qualifies for registration.
A foreign child adopted by Nigerian parents can equally become a citizen through registration.
Therefore, when an alien acquires citizenship, it is done through the method of registration.
Answer:C. Registration