Which weapon was a distinctive feature of World War I utilizing poison that was later declared a war crime to use after WW1? A machine gun
B mines
C chemical weapons
D trench warfare​

C. Chemical weapons.

Explanation:

TL:DR Chemical gas is a war crime due to the amount of suffering it causes. Mustard gas burns the skin, Phosgene is impossible to see, and Chlorine can make you drown in your own body.

Machine guns had been in use and invented since 1908, with invention of weapons such as the Vickers and German Maschinegewehr 08. Trench warfare wasn't a weapon, but more of a tactic. Mines weren't a war crime either as they most likely weren't deemed to cause "unneeded suffering" and their purpose usually was to simply discourage the travel of an area. Usually, if you stepped on a mine you'd die.

Meanwhile, Chemical gas was simply invented on accident as the German Empire's scientist,

i think it was   tsar alexander ii