Final answer:
Nationalism is accurately described as intense national pride and unity, and it often places a nation's interests above those of others. This sentiment was particularly influential in 19th century Europe and is not exclusively linked to economic prosperity or decline.
Step-by-step explanation:
Among the statements provided about nationalism, the one that is true is that it can be best described as an intense feeling of national pride and unity. Nationalism is a political ideology that emphasizes the interests of a nation, often exalting one nation above all others and advocating for the promotion of its own culture and interests. This sentiment was especially prevalent in Europe during the 19th century, where it sometimes led to competition and conflict as nations and ethnic groups sought power, autonomy, and global recognition.
Nationalistic feelings can indeed intensify when a country feels threatened by outside forces or during times of economic decline, as people may rally around national identity as a source of strength and solidarity. However, nationalism also flourishes in economically prosperous countries when they pursue policies and cultural expressions that reinforce their national identity. Lastly, while nationalism can sometimes affect foreign investment and international relations, it doesn't inherently lead to a loss of control over foreign investment.