Answer & Explanation
D) cooperate on law enforcement.
When the European union formed nations began to cooperate on law enforcement. The European Union is set up with the aim of ending the frequent and bloody wars between neighbours, which culminated in the Second World War.
As of 1950, the European Coal and Steel Community begins to unite European countries economically and politically in order to secure lasting peace. The six founding countries are Belgium, France, Germany, Italy, Luxembourg and the Netherlands. The 1950s are dominated by a cold war between east and west.
The historical roots of the European Union lie in the Second World War. Europeans are determined to prevent such killing and destruction from ever happening again.