Radiation can cause a fire to spread by __________.

Correct Answer: Option D — the transfer of heat across an unobstructed space

Radiation is the transfer of heat energy through electromagnetic waves across an unobstructed space. Unlike conduction or convection, radiation does not require a medium, allowing a fire to ignite distant combustible materials simply by direct line-of-sight exposure, necessitating the use of fire shields or cooling water curtains.