Which of the following is a characteristic of a rollover?

A rollover is characterized by the accumulation of combustible gases at ceiling level before they ignite, making option C the correct choice. This phenomenon occurs when hot gases produced during a fire rise and collect near the ceiling. Once the concentration of these gases reaches a sufficient level and the right conditions are met, they can ignite, creating a visible flame rolling across the ceiling, which is a dramatic indication of changes in fire dynamics within a room.

In contrast, other options address different stages or effects of fire behavior. The occurrence of rollover is not specifically tied to the decay stage, as that stage generally refers to a period when the fire is burning low on fuel. While gases can ignite within a room's contents, this is not a defining characteristic of rollover itself, which specifically focuses on the behavior of gases at ceiling level. Furthermore, the option regarding flames being extinguished quickly does not relate to the rollover phenomenon, as rollover often indicates a more intense fire condition rather than a quick extinguishment of flames.