Final answer:
The correct statement is that under the national treatment rule, imported goods must receive the same treatment as domestic goods within the importing country, based on the principles of the World Trade Organization.
Step-by-step explanation:
The correct statement among the ones provided is: Under the national treatment rule, once a good or a service is legally present in a country, it must receive the same treatment as a domestic good or service. This rule is based on the World Trade Organization's principles that stipulate foreign and domestic goods should be treated equally once the foreign goods have met certain criteria, such as safety standards, which must be based on science. This creates a level playing field between domestic and imported products. The most-favoured-nation rule indeed requires that trade benefits, such as low tariffs, granted to one member country must be extended to all members, but this doesn't specify tariff equality in all situations.