All of the following weapons used in world war i had not been used in war before except:
a) the maxim gun
b) hand grenades
c) poisonous gas
d) tanks

The only weapon that was used before World War 1 was the maxim gun. The maxim gun is the earliest example of a recoil firing mechanism found in a gun. It was developed in 1883 and the first prototype was shown in 1884. Trials showed that it could fire 600 rounds per minute. The maxim gun is also known as the weapon that was used most by the British when fighting various colonial wars. 

Hand grenades, poisonous gas and tanks were all used for the first time in World War 1.