Correct Answer: C
Explanation:
Monochromatic light is one which is composed of single wavelength.
Light is made of photons, which are energy packets. And these photons have a specific wavelength.
Photons emitted from Sodium [through excitation - de-excitation process] is characterized by 530nm wavelength [yellow light]. And since in a sodium vapor lamp, the wavelength is approximately that of yellow light, the light is monochromatic in nature.