Final answer:
A common market is established when a group of countries join together to eliminate trade barriers, allow free investment and movement of workers across borders. Therefore, the correct option is d. a common market
Step-by-step explanation:
When a group of countries join together to eliminate duties and other trade barriers, allow companies to invest freely in each other's country, and allow workers to move freely across borders, what has been established is a common market. A common market requires a common external trade policy in addition to free trade within the group. An example of a common market is the European Union, which has both free trade and a common trade policy among its member countries.