Concept: All forms of economic integration aim to unite member countries in order to expand their market and promote trade among themselves.Explanation: Economic integration removes trade barriers such as tariffs and quotas between member states. This action increases the flow of goods and services among the nations. As a result, producers gain access to a larger combined market. The free movement of factors of production is present only in deeper forms like a common market. A common currency is present only in a monetary union. A common agricultural policy is not shared by every form of integration. However, widening the size of the market is a benefit common to every stage of economic integration.Answer: D. the size of the market is widened