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

The term "knockdown" specifically refers to the point in firefighting when the flames have been significantly reduced or extinguished, indicating that the fire has been effectively contained or subdued. Achieving a knockdown is often a crucial milestone in firefighting efforts, as it allows firefighters to focus on further suppression and control operations without the immediate threat of flames.

In the context of firefighting terminology, other terms like "water on the fire" implies that water is being applied to extinguish it, but it doesn't necessarily mean that the fire has been subdued or darkened. "Ventilation complete" pertains to the process of removing smoke and heat from a structure to improve visibility and conditions for firefighting and rescue operations. "Primary search complete" signifies that firefighters have searched for and possibly rescued any victims, but this does not inherently relate to the fire's status in terms of being subdued or darkened. Therefore, "knockdown" is the correct terminology that denotes the effective reduction or extinguishment of a fire.