The correct answer is: "The adoption of a common currency in the European Union".
There are two tendencies about how the European Union should develop: widening and deepening.
The concept of deepening is referred to the process of creating a progressively closer union, and is visible in many integration policies adopted by the EU. For example, one of these policies could be the adoption of a common currency which supposes a big step forward in the establisment of interdependencies among the member states and common strategies.
On the contrary, the strategy of widening consists on including more members in the union and growing but with looser relationships than those promoted by the defenders of deepening.