The correct answer is D.
The D-Day invasion, also known as the Normandy landings, took place in June 1944 in mainland Europe, in the nazi-ruled French region known as Normandy. The allied forces aimed to re-open the Western front of WWII, and to push back the nazi Germans by reconquering first France (which was under their dominance) and then by starting gaining new territories, constantly coming closer towards Berlin.
This operation was a turning point in the war. From this moment on, the US and Western European allied forces (what was left from UK and France) started to fight the nazis on the West, while the URSS, that has also intervened in the conflict, did the same from the East.
