Hi there! I can't say I'm an expert, but one of my college majors is History, so I hope I can hope a bit. There are several good answers, WWI is often known as the great trench war due to the enormous trench structure that was built across the European front. It's also sometimes been called the first modern conflict due to the introduction of vast new weapons and military strategies. However, I think that because the question uses the words 'complete mobilization' and when we consider the huge scale that WWI was fought on, we can conclude that the best answer is C, "Total war".
Hope that helps a bit!
