Answer:
1940s
Step-by-step explanation:
Although it was recognized before the dates you provided, the mechanism wasn't fully understood. In the 1800s, people made comments on the connection between hacking blood, TB rates, and early deaths. Remember that until the 1900s, medicine was not a science, and it was difficult to determine what caused any sickness, including cancer. After Mendel was rediscovered and Watson/Crick et al. were still being discovered, DNA mutations were only hazily understood. Additionally, consumption skyrocketed once the US government included cigarettes in GI rations; I doubt that as many individuals smoked or chain smoked earlier.
The finding was discovered in the 1930s by German scientists. The findings, however, went unnoticed for 20 years until being found in the USA and other parts of Europe since the rest of the world rejected them all as Nazis and did not want anything to do with them.