Answer: B. Regional trade agreements may restrict trade from outside of the regions in the agreement.
Explanation:
Regional trade agreement is a form of trade agreement usually between two or more countries in a particular region which will allow for easy movement of goods between the borders of that particular region. Examples are North American Free Trade Agreement, European Union etc.
If an economist is a proponent of free trade amongst nations, the economist will be worried that the proliferation of regional trade agreements may restrict trade from outside of the regions in the agreement. This is because there'll only be free trade for the countries that are in that particular region.