Answer:
Radio waves
Explanation:
The electromagnetic spectrum consists of waves having different wavelengths. Wavelength of a wave is defined as the distance between two consecutive crests and troughs. It is inversely proportional to the frequency of the wave.
Out of the given options, the wave having longest wavelength is radio waves. Its wavelength is in between 1 mm to 10,000 km.
So, the correct option is (a) " radio waves ".
