What is the role of "World Trade Organisation" in international trade?

all of the above

WTO is expected to establish a rule-based trading regime in which nations cannot place arbitrary restrictions on trade. In addition, its purpose is also to enlarge production and trade of services, to ensure optimum utilisation of world resources and to protect the environment. The WTO agreements cover trade in goods as well as services to facilitate international trade (bilateral and multilateral) through removal of tariff as well as non-tariff barriers and providing greater market access to all member countries.

The role of WTO is thus to ensure that trade flows as smoothly, predictably, efficiently and as freely as possible.