Which industry had the greatest impact on the economy in the 1920s?

The correct answer is: Car and Automotive

The 1920s saw a huge rise in the production and adoption of cars in the United States.

Led by Ford's Model T, cars increasingly became cheaper and more available to wider public.

It was the single biggest revolution of personal transport in the history of mankind.

Hence, this radical change and shunning of animal driven carriages gave rise to the industry of cars and automobiles and became the largest factor in the United States economy.

Car/automotive industry had the greatest impact on the economy in the 1920s, I believe.