Final answer:
The European Union facilitated a single currency, the euro, for economic integration among its member states, with the European Central Bank overriding individual monetary policies.
Step-by-step explanation:
The European Union (EU) has largely achieved the reality of a single currency and a regional central bank with the introduction and adoption of the euro. Following efforts to integrate economies and avoid conflict after World War II, EU nations began with a free trade association, moving through stages to finally become an economic union. The euro serves as a common currency, replacing most national currencies, such as the German mark and the French franc. The European Central Bank, with representatives from all euro nations, determines monetary policy for the eurozone. However, some nations like the United Kingdom (before Brexit), Denmark, and Sweden have chosen to keep their individual currencies.
Although the adoption of a merged currency, the euro, enhanced economic integration among EU countries, it also means that individual nations surrendered control over their domestic monetary policies. For example, when the European Central Bank makes a decision that may not align with the individual needs of a country such as Portugal, it reflects a situation where the nation's interest rates and monetary policy are dictated by a centralized authority.