Under a "yellow-dog" contract, .
a. employees pledged to refrain from union membership
b. employees had to pay union dues whether or not they were union members
c. employers had to hire employees referred by the union
d. employees had to accept union membership as a prerequisite for employment

Yellow dog contract refers to those contracts, that take place between employer and employees which contains clauses relating to anti-union agreements.

Explanation:

Option A is the correct option that perfectly relates to yellow dog contracts. Employees in an organisation are restricted from entering into any agreements that relates to joining of any union. An employee agree to not to join or enter into any agreements relating to unions and if they do so, they shall resign before joining. they are not allowed ro enter into any such agreements.