The correct answer is D. Custom duties
In 1957 the members of the European Coal and Steel Community (Italy, France, West Germany, Netherlands, Belgium, and Luxembourg) signed the treaty of Rome, by which the European Economic Community was created. The main goal of this international organization was to boost the political and economic cooperation among its country members, hoping to reduce tensions and promote reconciliation. The EEC was designed to create a common market through the elimination of customs duties and several commercial barriers and by establishing a common international commerce policy among its members.