The D-Day invasion, officially known as Operation Overlord, had a significant impact on the outcome of World War II. The most accurate answer to your question is:
b) it allowed Allied troops to advance into Germany once France was free.
The successful invasion of Normandy on June 6, 1944, allowed Allied forces to establish a strong foothold in Europe. This led to the liberation of France and ultimately made it possible for Allied troops to push into Germany from the west, contributing to the eventual defeat of Nazi Germany in May 1945.
Options a) and c) are not entirely correct in the context of D-Day's impact. While the invasion did affect the overall strategy of the war, it didn't directly relate to fighting with Soviet armies or forcing Germany to focus on the Eastern Front in a simple manner. Option d) is also incorrect, as D-Day was aimed at liberating Western Europe from Nazi control, not providing Hitler with means to invade Italy.