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Fire has distinct phases of combustion, and one of them is smoldering—the slow, flameless burning of solid fuels. In this stage, heat continues to be generated by the oxidation inside the material, and you’ll often see smoke with little or no visible flame. Smoldering can linger after flames die and can reignite if fresh air enters, which is why recognizing it is crucial for suppression and ventilation.

That’s why the answer is the best fit: it identifies a true stage of fire characterized by flameless, slow burning, unlike cooling (not a fire stage) or boiling (not how fire behaves), and while incipient refers to ignition, smoldering specifically describes this flameless combustion phase.