Final answer:
The airplane caused the fewest casualties in World War I. Artillery was the leading cause of death, whereas airplanes were less effective in causing direct casualties and were often used for reconnaissance.
Step-by-step explanation:
The weapon that caused the fewest casualties in World War I was the airplane. While the development of machine guns, artillery, and tanks significantly increased the lethality of battle, causing devastating casualties on all sides, the use of airplanes in combat was still relatively new and less effective in causing direct casualties. Artillery, being incredibly accurate and having a vast range, was the leading killer during World War I. Machine guns significantly changed the nature of warfare with their ability to fire hundreds of rounds per minute. However, airplanes, in their infancy in terms of military use, were more frequently used for reconnaissance than as effective weapons of war causing mass casualties.
The introduction of the tank by the British was also a significant development in World War I military technology, but it did not result in the highest number of casualties. On the contrary, it was major battles such as Verdun, with extensive use of artillery shelling, that resulted in the highest number of deaths.