What significant feature did the Chinese develop in the first century AD related to military use?

The significant development by the Chinese in the first century AD that is closely related to military use is a form of gunpowder. This innovation marked a pivotal moment in military technology, leading to more advanced methods of warfare and weaponry.

Gunpowder, which consists of a mixture of saltpeter, charcoal, and sulfur, enabled the creation of explosives and propellant for projectiles. Its military applications transformed combat strategies and the effectiveness of armies, as it allowed for the development of explosive devices, early firearms, and artillery. This advance revolutionized warfare not only in China but also influenced military tactics globally as the knowledge of gunpowder spread over centuries.

In contrast, while the development of a rocket propulsion system is relevant to military technology, it came later and is not as directly associated with the first century AD. Manned flight technology and space exploration methods were not developed until many centuries later, well beyond the first century. Thus, the choice of gunpowder distinctly aligns with the historical advancements in military capabilities during that period.