Which of these have played the most important role in establishing and maintaining the strong sense of national identity held by the people of France?

the French Revolution

the Roman conquest

language and culture

Bordeaux and the Riviera

It would be primarily the "French Revolution" that has played the most important role in establishing and maintaining the strong sense of national identity held by the people of France, since this revolution did away with the monarchy and ushered in an era of (relative) governance eventually.