The D-Day invasion in June 1944 was important to the outcome of World War II because it Opened a new Allied front in Europe
Avoided use of the atomic bomb against civilian targets
Forced Italy to surrender
Stopped Soviet advances in eastern Europe

Opened a new Allied front in the Europe

Explanation:

There was only one front after the fall of France which were the Soviet. Soon after another front was created in Italy, where Italy negotiated for peace meaning another front was created. Then the success of the D-day, landing allowed for a 3rd front to open up, which spread the German soldiers very thin.

Soviet was a de facto ally of the Entente so it cannot be the fourth one. Italy only surrendered because the war was growing unpopular in Italy and Benito was toppled from power. They also surrendered as the Entente had got a holding of Sicily so it cannot be the third one. It cannot be the second one, as I am pretty sure that the Luftwaffe would have prevented this from ever happening

2018 means 2018 ce or common era

there have been other eras too including bce, which included way more years.