The correct answer is "no barriers to trade from nation to nation."
The definition of a common market is "no barriers to trade from nation to nation."
A common market is basically a free trade region or area, where countries can freely trade goods and services. This allows the participant countries to have good commerce treaties with no barriers and minimum intervention of the government. The best example could be the European Union or NAFTA, the North American Free Trade Agreement between Mexico, the United States, and Canada.