Answer:
C. "The US is the best country in the world and has the best language, government, and culture."
Step-by-step explanation:
One of the main characteristic of supporters for nationalism ideas is the thought that their country is superior over another country. They tend to be willing to support any programs as long as it benefit their own country, even if those programs will create a determinantal effects for other countries.
While being proud from being a citizens from a certain nation is not inherently bad, developing a thought that people from other nations somehow superior might create hostility that resulted in direct conflict. Events such as world war I and II are prime examples on why extreme nationalism could lead to fatal disasters.