What is a requirement for backup lines during an offensive attack?

Having a separate water supply for backup lines during an offensive attack is crucial for ensuring that operations can continue effectively even if the primary attack line encounters issues. This requirement helps prevent any potential disruptions in water flow, which could occur if the main supply is compromised or becomes insufficient due to the demands of the firefighting efforts. By having an independent water source for backup lines, firefighters can maintain their operational capabilities, enhance safety, and improve overall response effectiveness.

The other options do not adequately address the operational needs during an emergency. For instance, a requirement stating that backup lines must be smaller than the main attack line does not consider the possibility of needing the same flow rate or pressure for effective firefighting. Additionally, relying on the same water source as the attack line might introduce risks if that line fails or gets obstructed. Lastly, while the storage location for backup lines is important, it does not address the critical functional aspect of ensuring a reliable water supply during an emergency scenario, which is the primary focus of the requirement.