Final answer:
John L. Lewis told workers "The president wants you to join a union" to increase union membership and gain more authority.
Step-by-step explanation:
John L. Lewis told workers "The president wants you to join a union" to increase union membership and gain more authority (option a). The passage of the Wagner Act in 1935 provided federal support for unions and workers' right to bargain collectively, leading to a rapid expansion in union membership. By associating union membership with the president, Lewis aimed to encourage more workers to join the union and strengthen the labor movement.