Final answer:
Radio waves have the longest wavelengths and the lowest frequency in the electromagnetic spectrum, which means they also have the least energy compared to other types of solar radiation.
Step-by-step explanation:
The portion of the electromagnetic radiation emitted by the sun that has the least energy is d) Radio waves. This is because in the electromagnetic spectrum, energy and frequency are directly proportional, while the wavelength is inversely proportional to energy.
Radio waves have the longest wavelengths and the lowest frequency, which means they also have the least energy compared to other portions of the spectrum such as infrared, visible light, or ultraviolet radiation.
Regions of the spectrum like ultraviolet radiation can be harmful to living tissues, and photons in that higher-energy ultraviolet region are more effective at accelerating a solar sail. Conversely, low-energy radiation such as radio waves is not harmful and is found at the low-frequency end of the electromagnetic spectrum, far away from the higher energy regions.