Final answer:
Frederick most likely sued for his civil rights under the Civil Rights Act of 1866 or the Fourteenth Amendment, both of which protected citizens' rights, including those of African Americans.
Step-by-step explanation:
Frederick might have sued for his civil rights under several possible codes, but the most significant legal frameworks at the time would have been the Civil Rights Act of 1866 and the subsequent constitutional amendments. The Civil Rights Act of 1866 was a landmark legislation that was the first federal attempt to define citizenship and protect the fundamental rights of Americans, specifically including African Americans.
This act, along with the Fourteenth Amendment, which guaranteed equal protection under the law, were crucial in providing the legal grounds for African Americans to challenge segregation and discrimination.