Answer:
D. the process of blending many ethnicities in the United States into a single nationality
Step-by-step explanation:
"Historically, it is often used to describe the cultural integration of immigrants to the United States. ... The exact term "melting pot" came into general usage in the United States after it was used as a metaphor describing a fusion of nationalities, cultures and ethnicities in the 1908 play of the same name." (Quote from Wikipedia).