A boundary that prohibits fires from crossing is established between what?

The correct choice pertains to the relationship between converging friendly surface forces. In military operations, particularly in fire support coordination, it is crucial to establish boundaries to prevent friendly fire incidents. When you have friendly surface forces that are converging toward a common objective or area, a boundary is established to ensure that any fires, whether from artillery, air support, or other means, do not intersect in a way that could endanger friendly troops. This kind of boundary is designed to facilitate coordination and enhance the safety of operations, allowing each force to operate effectively without risking harm from their own side.

The other options, while they involve boundaries or restrictions in military context, do not specifically capture the nuance of preventing fires from intersecting between friendly units. For example, while a boundary between friendly and enemy forces is crucial for engagement rules, it does not focus on friendly fire prevention in a converging scenario. Ground and air forces operate under separate coordination structures, and layered defense systems typically involve overlapping defensive measures rather than prohibitive fire boundaries among forces. This is why the establishment of boundaries is especially relevant in scenarios involving converging friendly surface forces.