The correct answer is A) convection. Closed doors to a stair tower help prevent the spread of fire by convection. Convection is the transfer of heat through the movement of a fluid, such as air or smoke. When a fire occurs, the hot gases and smoke will rise and spread through the building via convection. Closing the stair tower doors creates a barrier that helps contain the convection of heat and smoke, preventing it from spreading to other areas of the building. The other answer choices are incorrect because they do not adequately explain how the closed doors prevent the spread of fire. Conduction is the transfer of heat through direct contact, radiation is the transfer of heat through electromagnetic waves, and ventilation refers to the circulation of air, which would not necessarily contain the fire's spread.