Question

which has the longest wavelength and, therefore, the lowest frequency/energy
radio waves
visible light
infrared rays
gamma rays

Explanation:

In the electromagnetic spectrum, radio waves have the longest wavelength, and according to the relationship $c = \lambda
u$ (where $c$ is the speed of light, $\lambda$ is wavelength, and $
u$ is frequency), longer wavelength means lower frequency. Also, energy $E = h
u$ (where $h$ is Planck's constant), so lower frequency implies lower energy.

Answer:

Radio Waves