The correct answer is - c. when Germany invaded Belgium in a strategy to quickly defeat France.
The first major battles in World War I occurred when the Germans in a quick and well planed action attacked and invaded Belgium. The reason for the invasion of Belgium was that through the Belgian territory, the Germans would have had much better chance of defeating the French troops. If the Germans attacked France directly from their territory, the geography of the area would have made a lot of problems, while through Belgium, the terrain was much more open, and on top of it, the French troops were not expecting an attack from that side, thus they had much less soldiers in that part of the country.