Which military product development breakthrough was specifically created to break the stalemate of trench warfare during World War I?

Throughout history, military technology has driven innovation and changed the course of warfare. Product development in military contexts often leads to revolutionary designs that transform battlefield tactics and strategy. This poll tests your knowledge about a significant product development breakthrough that changed military history. Consider the research, design challenges, and ultimate impact of these military innovations before selecting your answer.

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The tank (first deployed by Britain as the Mark I in 1916)
The machine gun (Maxim gun, developed in the 1880s)
Poison gas weapons (first used at Ypres in 1915)
The fighter aircraft (developed from reconnaissance planes from 1914-1915)