Answer:
Correct answer is Stowaway.
Step-by-step explanation:
The answer to this question is pretty simple. This is the only illegal way to obtain citizenship, as it is prohibited.
Naturalization is a pretty common process, and it usually means that you need to fulfil certain conditions.
Birth is a common way also, and in most of the countries if you are born in in, you will get citizenship by default.
Also if you get married to a citizen of certain country with certain requirements you can apply for citizenship also.