The correct answer is (a) Retortion.
Retaliation is a measure taken by a state in response to a wrongful act by another state. It is a form of self-help, and it is not considered to be a peaceful means of settlement of dispute under International Law.
Conciliation is a process in which a neutral third party helps the parties to a dispute to reach a settlement. It is a peaceful means of settlement of dispute, and it is often used in cases where the parties are unable to reach a settlement on their own.
Negotiation is a process in which the parties to a dispute discuss the issues in dispute and try to reach a settlement. It is a peaceful means of settlement of dispute, and it is the most common means of settling disputes between states.
Good offices is a process in which a third party offers its services to the parties to a dispute in order to help them to reach a settlement. It is a peaceful means of settlement of dispute, and it is often used in cases where the parties are unable to negotiate a settlement on their own.