series of colors of the rainbow. list of radioactive elements. band of frequencies of light waves. range of conductivity of metals.
I gave my students this multiple choice question: The electromagnetic spectrum is described as a

Only one answer is correct among these 4 choices:
- series of colors of the rainbow.
- list of radioactive elements.
- band of frequencies of light waves.
- range of conductivity of metals.

Could you tell me which one is correct?

band of frequencies of light waves.