In the event of a fire, the doors to a stair tower must be closed to prevent the spread of fire by ____________.

Correct Answer: Option D — convection

Convection is the transfer of heat through the movement of air or liquids. In a fire, hot gases and smoke rise and circulate through stairwells. Closing fire doors prevents this convective heat transfer, effectively compartmentalizing the fire and protecting escape routes from smoke and thermal spread.