From the Super Mario Wiki
The Eternal Flame is an item in Mario is Missing! It is lit at the Tomb of the Unknown Soldier located at the base of the Arc de Triomphe in Paris. It is stolen by Bowser's Koopa Troopas, causing the entire Arc de Triomphe to shut down. The Koopa Troopas then roam around the city until Luigi defeats them and takes it back. After asking the locals about the item, with all of them providing information such as how the Arc dates back to the 1790s (which is misleading; although one of its sculptures, "La Marseillaise", depicts an event from 1792, the Arc was not completed until 1836) , he is asked a series of questions by the monument's Curator before he can return it. Afterwards, he receives a reward of $1750.