The success of the automobile industry in the 1920s was due to

Mass production and easy credit
Cheap gasoline and low government regulation
Monopolistic practices
Sophisticated advertisement campaigns

Mass production and easy credit.

Explanation:

Before the 1920s, cars were very expensive. In the 1920s, the industry turned to mass production because it was really more profitable to produce millions of financially affordable cars than producing a few thousands for the rich people. Technology advances such as the assembly line and design innovations, plus sales strategy  and easy credit, made this possible. In 1918, only 1 in 13 families owned a car. In 1929, 4 out of 5 families had a car.

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