Which of these military products was first introduced during World War I and fundamentally changed battlefield mobility?

Throughout military history, certain products and innovations have dramatically altered the landscape of warfare. Some were designed specifically for military use, while others were civilian products adapted for war. This poll tests your knowledge about a revolutionary product development that had profound effects on military tactics and strategy. Do you know enough about military product history to identify the correct answer?

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The M1 Garand rifle, the first standard-issue semi-automatic rifle for American forces
The military wristwatch, replacing pocket watches for better coordination of troop movements
The steel combat helmet (Brodie helmet/Adrian helmet), replacing cloth and leather headgear
The portable field radio (backpack radio), enabling real-time communication between advancing units