Answer:
b. Labor supply increases, and wages decrease
Step-by-step explanation:
It is important to note that labour unions fight for the right of their members by increasing wages and also serve as barrier to entry (restriction on supply of labour).
Since waiters have no union there would be a decrease in their wages as compared to bartenders who formed a union.
The supply of waiters increases because there is an increase in demand for waiters by employers because of a lower wage rate as compared to the wage rate of bartenders.