After the attack on Poland by Germany, Britain and France declared war on Germany, which was the immediate cause of the attack on France. This attack by Germany would certainly have happened, but this was only a trigger. The battle for France known as the fall of France began on May 10, 1940 and lasted for about six weeks. It was a German invasion of the Low Countries, where the nazi armored units crossed the Ardennes, along the Somme Valley, and thus cut across and encircled allied units that advanced to Belgium.
The answer is: C.
