Answer:
(D) A temperature difference and the presence of electromagnetic waves.
Step-by-step explanation:
In order for radiation to occur, there must be a difference in temperature for the thermal energy to be transferred, and there has to be electromagnetic waves.
Radiation = The transfer of thermal energy via electromagnetic waves
Since we know we need electromagnetic waves, only either option C or D could be correct.
Q: How do we know that radiation needs to have a difference in temperature?
Let's look at an example, such as the Sun and the Earth.
The sun is a star, a burning ball of gas. Because it is gas, it is less dense, and its temperature is way higher than the mainly solid and liquid Earth.
A: Heat from the sun (thermal energy from the sun) is being transferred to the Earth via radiation, and for thermal energy to transfer (or become heat), there has to be a difference in temperature.
More below:
+ In order for thermal energy to transfer in the first place, it has to become heat.
Heat ONLY transfers from warmer objects to cooler objects, meaning that there has to be a warmer object and a cooler object, or a "difference in temperature" in order for something like radiation to occur.