The least aggressive tactic used by advocacy groups would typically be distributive bargaining (option c).
Distributive bargaining refers to a negotiation strategy where conflicting parties try to reach an agreement by dividing a fixed amount of resources. It involves seeking a compromise and finding a middle ground between the parties involved. While it may still involve advocacy and negotiation, distributive bargaining is generally considered less confrontational and aggressive compared to other tactics such as public communications (option a), media advocacy (option b), or class action lawsuits (option d), which can involve public pressure, media exposure, or legal action respectively.