Final answer:
The military symbol for a mass-detonation hazard is represented by a square. This symbol indicates that if one item detonates, it may cause a chain reaction with other nearby items.
Step-by-step explanation:
The military symbol for a mass-detonation hazard is the square (option d). This symbol is part of a set of standardized icons used by the military to categorize various types of ordnance based on the potential hazard they pose. A square symbol indicates that the ordnance can cause a mass detonation, which means that if one item detonates, it is likely to cause other nearby items to detonate as well. This is an essential piece of information for handling and storing military munitions safely.