I believe the answer is: B.That it makes it possible for society to become better off by increasing both its production and its consumption.
Without trades, in order to fulfill all needs of the people, a country need to separate their time and resources to produce each of the needed products. With trades, a country could increase the production of the products in which they have a natural advantage at, and trade the products with other countries in case we need different product that we do not produce here.