The correct answer is D. Collective bargaining means bargaining with workers who belong to an union.
According to the International Labor Organization (ILO), collective bargaining is the voluntary process that takes place between workers 'associations (usually, though not always, gathered through a union or group of unions) and employers' associations. The purpose of the negotiation is to reach an agreement regarding the labor conditions applicable to the generality of workers in the area in which the negotiation is circumscribed.
Sometimes, as a measure of pressure for negotiation and to enforce agreements when they consider that they have not been complied with, workers can go on strike.