Final answer:
The common currency of most of the countries of the European Union is d) the euro.
Step-by-step explanation:
The common currency of most of the countries of the European Union is the euro. The euro was introduced in 1999 and has been widely accepted by EU members. Great Britain is a major economic player in the EU but has retained the pound sterling as its currency. Many other countries, both within and outside the EU, have also adopted the euro as their national currency.