guaranteed that all people born or naturalized in the United States were citizens and that no state could restrict their rights

The correct answer is the 14th Amendment.

It states that "All persons born or naturalized in the United States and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside. No State shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States; nor shall any State deprive any person of life, liberty, or property, without due process of law; nor deny to any person within its jurisdiction the equal protection of the laws."

The Fourteenth Amendment to the Constitution of the United States ('XIV Amendment') is one of the post-Civil War amendments, and includes, among others, the Due Process Clause and the Equal Protection Clause. It was proposed on June 13, 1866, and ratified on July 9, 1868.

The amendment provides a broad definition of national citizenship, which overrides the decision of Dred Scott v. Sandford (1857), who had excluded slaves and their descendants, from possessing constitutional rights. It requires states to provide equal protection before the law to all persons (not just citizens) within their jurisdictions. The importance of the Fourteenth Amendment was exemplified when it was interpreted to prohibit racial segregation in public schools in the Brown v. Case. Board of Education.

The other two post-war amendments are the Thirteenth Amendment (prohibiting slavery) and the Fifteenth Amendment (it prohibits qualifications regarding voting determined by race).