The correct answer is:
a. The blending together of cultures in America due to high immigration numbers in the late 19th century.
During the Gilded Age, which generally refers to the late 19th century, the United States saw a significant influx of immigrants from various countries. This period marked a time when diverse cultures began to mix and influence each other, leading to the concept of America as a "melting pot" where different ethnicities and cultures can coexist and integrate into a single society.