The correct answer is C) Germany's invasion of neutral Belgium was not going to be ignored.
The incident occurred that caused an international uproar and for Britain to join the war was "Germany's invasion of neutral Belgium was not going to be ignored."
The German army had to cross Belgium. But, of course, Belgium being a neutral country in World War 1, did not allow German troops to cross through its territory. Germany chose the easy way, to invade Belgium on August 4, 1914. On August 2, the German government decided to send an ultimatum to the government of Belgium. The German troops had to cross Belgium. Period. Belgium refused with the support of Great Britain and Germany entered Belgium territory initiating the Battle of Liege.