Which of the following is usually a sign of a fire that is entering a fully-developed phase?

High heat production is a key indicator that a fire is entering a fully-developed phase. During this phase, the fire has reached its peak intensity, consuming available fuels and significantly raising ambient temperatures. The increase in heat production is often associated with the combustion of all available combustibles in the vicinity, indicating that the fire is actively consuming materials and potentially spreading. This intense heat can also create life-threatening conditions for individuals in the vicinity, as it can compromise structural integrity and escalate the risk of flashover.

In contrast, the absence of visible flames can sometimes occur during certain conditions of fire, where flames might not be externally visible despite high heat levels being present. Colorless smoke usually suggests that combustion is incomplete and does not necessarily correlate with a fully-developed fire, as it may indicate conditions that could still be in an earlier phase of development. Rapid oxygen depletion typically occurs as a fire burns and consumes oxygen from the space; while it is a serious condition, it does not solely signal the transition to a fully-developed fire since various factors can contribute to oxygen levels. Thus, high heat production stands out as the most definitive sign that a fire has progressed into its fully-developed phase.