Which of the following denotes that the fire has been effectively darkened?

The term that denotes that the fire has been effectively darkened is "knockdown." In firefighting terminology, achieving knockdown refers to the significant reduction of fire intensity, meaning that the flames have been largely suppressed or extinguished in the area where firefighters are working. This is a critical point in the firefighting process, as it indicates that the immediate threat of the fire has been mitigated, allowing firefighters to focus on further extinguishing any remaining fire and conducting additional operations safely.

The other choices refer to different aspects of firefighting operations. For instance, water on the fire implies that water is being applied to extinguish the flames but does not necessarily indicate that the fire has been darkened or knocked down. Ventilation complete signals that airflow has been established, which is essential for smoke removal and thermal cooling but does not directly mean the fire has been subdued. Similarly, primary search complete pertains to the search for occupants within the structure, ensuring that people are safe and accounted for, rather than the status of the fire itself. Thus, "knockdown" is the precise term used to indicate that the fire has been effectively darkened.