Final answer:
Artillery was not a new method of warfare used during World War I; it was already in existence but was heavily developed during the war.
Step-by-step explanation:
The weapon that was not a new method of warfare used during World War I is artillery. Artillery existed prior to the Great War, and while it was significantly developed during this time, it was not newly introduced. In contrast, poison gas, airplanes, and U-boats (submarines) were indeed new methods of warfare that appeared during World War I. Poison gas was first used by German troops in 1914 and evolved throughout the war with various types being deployed, such as chlorine and mustard gas. Airplanes were initially used for reconnaissance but later for offensive actions such as bombing and strafing. The British introduced tanks, another new technology in World War I, which had not been used in warfare before.
During World War I, several new methods of warfare were introduced, including poison gas, artillery, airplanes, and U-boats. However, out of these options, artillery was not a new method of warfare used during World War I. Artillery guns had already been in use prior to the war, but their effectiveness and range were greatly increased during the war.