The decision to stop firing across the border was a unilateral one.

Answer: Option B

Unilateral : (of an action or decision) performed by or affecting only one person, group, or country involved in a situation, without the agreement of another or the others.
Collective : done by people acting as a group.
Bilateral : having or relating to two sides; affecting both sides.
Multilateral : agreed upon or participated in by three or more parties, especially the governments of different countries.
Multiple : having or involving several parts, elements, or members.

Antonym of Unilateral is Bilateral