Final answer:
The British failed to warn their own troops during the gas attack in September 1915.
Step-by-step explanation:
The mistake the British made when launching a gas attack in September 1915 was b) They failed to warn their own troops. This lack of communication resulted in their own soldiers becoming victims of the gas attack. Gas masks became a regular part of a soldier's kit, but without proper warning, many troops were unable to react in time.