Customs unions differ from free trade areas by the manner in member countries apply tariffs to non-members.
Option D
Step-by-step explanation:
"Custom unions and free trade areas" are very similar to each other in terms of arrangements of internal manner in which the member nations agree to trade among themselves. In the both custom unions and free trade the members help each other lower the hindrances to trade such as taxes, tariffs, and many more, hence creating less competition in the market. The main point of difference between custom union and free trade is how their approach towards non treaty nations is. In custom unions, the members are required to establish external tariffs where as in free trade, as the name suggests, members are free to trade and do not require any tariff.