Final answer:
The clearest and most accurate version of sentence 6 in the context given is 'the EU introduced the euro as its official currency', which effectively communicates the subject of the sentence and the action taken by the European Union.
Step-by-step explanation:
In the context provided, the best version of the underlined portion of sentence 6 is 'the EU introduced the euro as its official currency'. This phrasing is clear, concise, and correctly attributes the action of introducing the euro to the European Union (EU). It also places the subject, the EU, at the beginning of the sentence, making it easy to understand.
By 1999, the euro was established as a common currency to facilitate free movement of goods, services, money, and people within the EU. Although the euro was initially used for commercial and financial transactions, it quickly became the legal tender in most EU countries. However, not all member states adopted the euro, with notable exceptions like the United Kingdom, Denmark, and Sweden, which retained their own currencies.