The correct answer is D) War at Sea.
Although you did not specify what war you are referring to, we are going to assume that you refer to World War I.
The "front" that used torpedoes and submarines was the War at the Sea.
Germany used to have a powerful navy that disrupted the English Sea through the use of torpedos launched by its submarines. Germany had many U-Boats. One of these boats shot a torpedo and sank teh Lusitania ship. which was one of the many motives US President Wilson decided to ask Congress for a declaration of war against Germany. The other major reason was the interception of the Zimmerman telegram in which Germany asked Mexico for its support in the war.
