The correct answer is "American troops and supplies reached Europe."
The Battle of the Atlantic was the longest military campaign throughout the 2nd World War. It started in 1939 with the Allies' naval blockade on Germany and this country's subsequent counter-blockade, which meant numerous years of naval combat which ended with the defeat of Germany in 1945.
At the end of the battle, Germans were unable to stop the incoming flow of supplies to mainland Europe. This was the critical event that made the D-Day landings possible, as supplies were available for an eventual occupation of German-controlled territories.