An applicant is eligible to file if, immediately preceding the filing of the application, he or she:

1. must be at least 18 years old.
2. has been lawfully admitted for permanent residence (see preceding section);
3. has resided continuously as a lawful permanent resident in the U.S. for at least 5 years prior to filing with absences from the United States totaling no more than one year;
4. has been physically present in the United States for at least 30 months out of the previous five years (absences of more than six months but less than one year break the continuity of residence unless the applicant can establish that he or she did not abandon his or her residence during such period)
5. has resided within a state or district for at least three months.
6. must show that he or she has been a person of good moral character for the statutory period (typically five years or three years if married to a U.S. citizen or one year for Armed Forces expedite) prior to filing for naturalization.

If you meet above requirements, you can study 100 citizenship test questions and answers or 25 citizenship test questions and answers if you have been a permanent resident for 20 years and your are at least 65 years old or older.