All of the frequencies or wavelengths of electromagnetic radiation.

• A. Electromagnetic radiation
• B. Wavelength
• C. Electromagnetic spectrum
• D. Frequency

Answer: (C)

The main idea is recognizing the term that names the entire range of electromagnetic waves. The electromagnetic spectrum is the complete collection of all possible frequencies and wavelengths that electromagnetic radiation can have, spanning from radio waves with long wavelengths to gamma rays with very short wavelengths. This spectrum groups these waves in order of wavelength (or frequency), and it also helps connect their properties through the relationship c = λf, meaning as wavelength gets longer, frequency gets lower, and vice versa, with speed of light as the constant link.

Electromagnetic radiation refers to the phenomenon of waves themselves, not the full set of all possible wavelengths and frequencies. Wavelength and frequency describe a single wave’s characteristics, not the entire range. So the description of all frequencies or wavelengths points to the electromagnetic spectrum as the correct term.