Development of scientific method
In the end of the major case of Brown Vs. Board of Education,
the Supreme Court ruled that racial segregation in public education facilities was UNCONSTITUTIONAL. This being said, the answer to your question, is D.
It struck down segregation laws as unconstitutional.
A) One feature of nationalism that was present in many nationalisms of the aforementioned period was the construction of a national identity based on differentiation from some "Other".
B) The visit of the commodore Matthew Perry on Japan with steamships led to the Japanese investment in modernizing the country to avoid being colonized by modern nation-states such as the US and other European powers. Much of the construction of Japanese nationalism of the Meiji period has to do with the context of modern imperialism in the late 1800s.
C) The unification of Italy. Italy's unification in 1861 happened after a long period of growing nationalism marked by the creation of secret societies such as Young Italy that worked spreading arguments in favor of a unified nation-state that ultimately led to the success of the unification movement.
One good organizing topic for the post–Civil War era would be "Reconstruction," since this was the stage that took place immediately after the war, in which the North tried to "Reconstruct" the South in terms of its economy and social structure.
Hope this helps!!
~Lena~
A strong warrior is selected as a leader of the group