Filling N400 or N600 for Naturalization?


N400 or N600?
My fiancé has been a U.S resident for approximately 21 years (since 3/25/1997) and is currently 33 years old. She wants to apply for Citizenship. She migrated to the U.S. from Brazil with her mother. Her mother was naturalized as a U.S. Citizen on Feb 23rd, 2003 when my fiancé was 17 years old. In doing some research regarding which form to fill out, it seems that my fiancé is already a U.S Citizen by “Derivation of Citizenship and Naturalization of Parents”. This is what I found:
“A child may become a U.S. citizen through the process of derivation if one of his parents becomes a U.S. citizen via naturalization. However, at the time the parent becomes naturalized, the child must have a green card, be under the age of 18, and living with the naturalized parent to take advantage of these laws.

Any more insight into this would be greatly appreciated!