In the eighth century, Frankish leader Charles Martel stopped the advance of Muslims from Spain into France, thus, impacting the Christian Europe's relations with Muslim Spain as well as impacting the Islamic expansion.
C.) The House of Representatives
In Plessy v. Ferguson, the United States Supreme Court ruled that the doctrine of "separate but equal" was permissible under the US Constitution.
This was later overruled in Brown v. Board of Education.
The correct answer should be the constitution because nobody is above the constitution. There are rules there on how the government is chosen, how it is changed, how it is kept in check, and if something is unconstitutional it is banned instantly by courts, and the government can't do anything that goes against the constitution.
As the Cold War heated up in the 1950s, the United States made decisions on foreign policy with the goal of containing communism. To maintain its hegemony in the Western Hemisphere, the U.S. intervened in Guatemala in 1954 and removed its elected president, Jacobo Arbenz Guzman, on the premise that he was soft on communism.