8 C.F.R. § 320.2 : Aliens and Nationality — DEPARTMENT OF HOMELAND SECURITY — NATIONALITY REGULATIONS — CHILD BORN OUTSIDE THE UNITED STATES AND RESIDING PERMANENTLY IN THE UNITED STATES; REQUIREMENTS FOR AUTOMATIC ACQUISITION OF CITIZENSHIP — Who is eligible for citizenship?

(a) General. To be eligible for citizenship under section 320 of the Act, a person must establish that the following conditions have been met after February 26, 2001:
(1) The child has at least one United States citizen parent (by birth or naturalization);
(2) The child is under 18 years of age; and
(3) The child is residing in the United States in the legal and physical custody of the United States citizen parent, pursuant to a lawful admission for permanent residence.
(b) Additional requirements if child is adopted. If adopted, the child must meet all of the requirements in paragraph (a) of this section as well as satisfy the requirements applicable to adopted children under section 101(b)(1) of the Act.

8 C.F.R. §320.2