Multilateral trading system conducted by the WTO might be persumed as a quite confusing system dealing with pages long legal texts, numerous trade deals, tough negotiations with one hundred sixty-four member countries which are not similar in economic terms. Essentially, the multilateral trading system is run by five basic principles. It should be mentioned that these principles of the multilateral trading system have been the main guidelines since the GATT 1947. In other words, these principles are the main principles of both the GATT and the WTO:

1. Free Trade without Discrimination Principle: Free trade without discrimination principle is composed of two sub-principles: Most-Favored Nation Principle (MFN) and National Treatment Principle.
2. Freer Trade Principle: Obviously, the best way of encouraging international trade is to lower the trade restrictions. The GATT/WTO have been trying to abolish trade restrictions in order to achieve freer multilateral trading system via negotiations. In other words, the GATT/ WTO negotiation rounds have been the unique platform to reduce the existing trade restrictions and make the world trade freer. In spite of the benefits and positive welfare effects, reduction within the trade restrictions or in other words, trade liberalization, can be painful. Due to the adjustment requirements, abolishment of the trade restrictions might be time demanding. Thus, the GATT/WTO allows member countries to implement the required trade liberalization gradually, leading to a progressive trade liberalization. In this respect, least developed and developing countries are given longer periods to implement their necessary progressive trade liberalization.
3. Predictability Principle: This principle is about not to raise a trade restriction that is decreased or abolished before. The countries that decide to open up their markets bind their commitments. These commitments take the form of import tariff ceilings on commodities trade. In this case, the bound commitment refers to bound tariff. Countries can change their bindings if they take the assent of all the other GATT/WTO member countries. It means that they have to persuade them via companseting their probable losses. Although there is a probability to alter the bindings, it is not that easy to put it into practice.
4. Promoting Fair Competition Principle: Multilateral trading system is not just a free trade system but also a system of fair competition. Fair competition can be distorted by dumping and subsidies. The additional duties charged by the countries that are exposed to dumping and subsidized exports can be very unfair, as well. Promoting Fair Competition Principle helps in avoiding unfair anti-dumping duties and countervailing duties.
5. Encouraging Economic Development and Reform Principle: The multilateral trading system has various goals, rules and implementations. Nevertheless, the countries within the multilateral trading system are not homogeneous in economic terms. Thus, they should not be treated equally in adopting the rules and implementing the decisions of the system. Thus, least developed and developing countries are subject to a time flexibility in adopting and implementing the requirements of the multilateral trading system.

As also understood from the information given, all of the principles in the options are the main principles of both the GATT and the WTO, so the correct answer is E.