What must be evaluated if the lines supplying a single flow have different diameters and lengths?

The correct understanding revolves around the fact that different diameters and lengths of supply lines significantly affect the pressure drop (DP) along those lines. Even if the lines are carrying the same volume of water, the size and length will cause variations in friction loss, which leads to differing pressure drops.

When water flows through a pipe, friction losses are determined by both the diameter and the length of the pipe; a longer line or a line with a smaller diameter will inherently create more resistance to flow, resulting in a greater pressure drop. Therefore, while these different lines may deliver the same flow rate, the actual pressure at the discharge end can be notably different due to the hydraulic effects of their respective characteristics. This principle is critical for ensuring adequate pressure is maintained throughout a firefighting operation and for effective scene management.

Considering the other potential answers, they fail to properly represent the realities of fluid dynamics in this context.