The European Union (EU) facilitates trade among its member countries primarily through the use of a common currency, specifically the euro, which is adopted by 19 out of the 27 EU member states. This common currency simplifies transactions, reduces exchange rate fluctuations, and encourages trade by making it easier for businesses to operate across borders within the Eurozone.
While the EU does employ tariffs and trade regulations with non-EU countries, within the EU's internal market, tariffs are generally abolished to promote free trade among member states. Export limits and military alliances do not directly pertain to the EU's internal trade framework.
So, the correct answer to your question about how the EU aids trade among its member countries would be the common currency.