To spur trade, four South American countries decide there will be no barriers to trade between the countries, agree on a common external trade policy, and allow factors of production to move freely between them. In short, the four countries formed a

A free trade area

Explanation:

A free trade area comes to existence when two or more countries sign an agreement that eliminates or reduces trade barriers among themselves. Usually, nations in the same region sign a free trade agreement that encourages economic cooperation.  In a free-trade area, goods and services can move from one country to another with minimal or no government interference in terms of tariffs, embargo, quotas, or other prohibitions.

The four South American countries have formed a free trade area. They have agreed to economic cooperation that allows free flow of trade among the nations. The countries have established a trading block that will spur economic growth in each of them.

it should be

answer; organization;