I believe the answer is: "African Americans increased their push for civil rights on the home front"
The sentiment was initially made by African Americans who were being blatantly discriminated by the white citizens, in both job fields, education, or even the usage of public properties. This became the reason for the push for civil rights which finally realised by the The Civil Rights Act of 1964
D) The newly formed Mexican government built an extensive network of trails and roads, allowing for easier migration by American citizens.
Although it is true that the Spaniards defended their borders very well, the fact is that America never tried to advance by force during the colonial government of Spain. The way in which America expanded to the west was slow and gradual, and as the United States grew in population and land, the new nation of Mexico was also modernized and its territories were populated, enriching, which aroused interest of the Americans by the territories of Texas, California, New Mexico.
I think the answer would be a
