Which constitutional amendment guarantees that all citizens receive equal protection under the law ?

Fourteenth Amendment to the United States Constitution

The 14th Amendment to the U.S. Constitution, ratified in 1868, granted citizenship to all persons born or naturalized in the United States—including former enslaved people—and guaranteed all citizens “equal protection of the laws.” One of three amendments passed during the Reconstruction era to abolish slavery and establish civil and legal rights for Black Americans, it would become the basis for many landmark Supreme Court decisions over the years.

Explanation:

four sitting presidents have been assassinated while in office but the first was abraham lincoln,