Answer:
adoption of a common external trade policy.
Step-by-step explanation:
The main difference between a customs union and a free trade area, is that a customs union establishes common tariffs for goods that are produced outside the union, while each member country of a free trade area establishes its own tariffs for goods produced outside the area.
For example NAFTA (or USMCA) is a free trade area, while the Mercosur is a customs union in South America.
*The European Union started as a free trade area, then it changed to a customs union, later into a common market, then a economic union, and now it is a political union.