Germany's poison gas in World War-I developed by

Haber was a German chemist who played a crucial role in the development of chemical warfare during World War I.

He is known for developing chlorine gas, one of the first poison gases used in warfare.

Fritz Haber led the German chemical warfare program and was instrumental in organizing the first large-scale use of poison gas at the Second Battle of Ypres in 1915.

While his contributions to chemistry (such as the Haber-Bosch process) were significant for agriculture, his role in chemical warfare remains controversial.

Therefore, the correct answer is Haber.