Answer: D) To freely trade with the bloc's other member countries.
Step-by-step explanation:
Free trade means there aren't any tariffs between any two countries. So any two countries can sell whatever goods they want without any extra taxes, fees, etc added on.
Choices A through C all describe situations in which trade is reduced/restricted between any two countries. This makes a nation more independent, but the drawback is that they aren't connected to the global stage as much.
Theoretically in an economic sense, a country is better off being connected more globally because it allows for specialization to happen and it lowers costs. Of course on a practical realistic level, things are more complicated but this is the general idea at least.