Proper attack line placement is _____ the key to success in reducing _____ time.

The correct phrase highlighting proper attack line placement is "Critical; reaction" because successful firefighting heavily relies on the efficient engagement of firefighting resources upon arrival at the scene. The attack line—used to suppress the fire—is fundamental to quickly and effectively managing the incident. Proper placement of this line can dramatically reduce the reaction time needed to suppress a fire, ultimately leading to a quicker containment and more effective operation.

In firefighting, every second counts, and the initial reaction to a developing fire can significantly influence the outcome. Thus, recognizing the critical nature of attack line placement highlights its role in tactical operations.

In contrast, other options do not convey the urgency or importance needed in a firefighting context. While terms like "essential" or "fundamental" carry weight, they do not encompass the immediate imperative that proper attack line placement represents in live-fire situations.