Which option correctly describes radiation?

• A. Heat transfer by electromagnetic waves emitted by a higher-temperature object and absorbed by a lower-temperature object.
• B. Heat transfer by conduction through solids only.
• C. Heat transfer by movement of a gas or liquid due to density differences.
• D. Heat transfer by chemical reaction.

Answer: (A)

Radiation is heat transfer by electromagnetic waves emitted by a warmer object and absorbed by a cooler one, and it can travel through empty space. This is why the Sun can heat the Earth without touching or needing a medium. The other modes involve matter: conduction requires contact and happens mainly in solids (though it can occur in liquids and gases too), convection moves heat through the bulk motion of a fluid, and heat from a chemical reaction describes energy released or absorbed during a process rather than a transfer method.