The correct answer is D. Reducing barriers to trade and travel.
Step-by-step explanation
The European Union is a political community made up of twenty-seven European countries that cooperate multilaterally mainly in the political and commercial sphere. Therefore, all members handle the same currency, the Euro, which allows them to trade and establish economic relationships without many barriers.
In addition, the policy of each country is characterized by being in harmony with the other members, allowing citizens' lives to be better in aspects such as tourism and travel, since they can freely transit through the member countries because they belong to one of them. Therefore, the correct answer is D. Reducing barriers to trade and travel.