The correct answer is A. collective.
A unilateral decision is one that is made by one party without the agreement or input of others. In the context of the question, the decision to stop firing across the border was made by one country without the agreement of the other country involved in the conflict.
A collective decision is one that is made by a group of people or organizations. In the context of the question, a collective decision to stop firing across the border would have been made by both countries involved in the conflict.
A bilateral decision is one that is made by two parties. In the context of the question, a bilateral decision to stop firing across the border would have been made by the two countries involved in the conflict.
A multilateral decision is one that is made by more than two parties. In the context of the question, a multilateral decision to stop firing across the border would have been made by more than two countries, such as the United Nations or the Organization of American States.
A multiple decision is not a valid option.