What must a country do to join the European Union? A. Establish Christianity as a state religion
B. Adopt a Socialist system
C. Reduce immigration
D. Have a stable democratic system

D. Have a stable democratic system

Explanation:

Countries in the European Union are expected to have high standards of democracy because this will guarantee the most welfare to their citizens.

It is also important because countries in the EU have free movement of citizens so they cannot have a situation where instability crosses to other EU countries.

This is therefore a major requirement to join and the reason why countries like Turkey have still not joined because the EU worries about the governance record of President Erdogan.