Which of the following enabled mass production in the 1920s? a. standardization of spare parts b. mass availability of electricity c. introduction of the assembly line d. new precision-manufacturing technologies.

The mass production is a way of the production of large amount of standardized materials and good. The same goes for the production of  the spare parts. By the year 1920, most of the goods that are produced in large amount are spare parts of the vehicles. Thus, the answer is letter A.