They were referring to The French Revolution.
Option: D
Step-by-step explanation:
Franco -Prussian war was happened in between France and Germany in the year of 1870. The Concordat of 1801 was an agreement signed between Napoleon Bonaparte and the representatives of both Rome and Paris. The Congress of Vienna had a meeting in 1814 between the ambassadors of European states.
The French Revolution happened in the year of 1789. The main cause of it was the financial or difference in wealth in between Britain and France. The national debt of France was no longer high like Britain. The treaty of Versailles was signed in the year of 1919.