which amendment extends due process protection to state acts?

Answer 1

The Fourteenth Amendment ... includes the Due Process and Equal Protection Clauses. It was proposed on June 13, 1866, and ratified on July 9, 1869.
The amendment ... requires the states to provide equal protection under the law to all persons (not only to citizens) within their jurisdictions.
... the true significance of the Amendment was not realized until the 1950s and 1960s, when it was interpreted to prohibit racial segregation in public schools and other facilities in Brown v. Board of Education.

"Section 1. 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."