MONOCHROMATIC LIGHT IS THAT LIGHT IN WHICH:
A. YELLOW AND RED LIGHT IS PRESENT
B. VARIOUS WAVELENGTHS IS PRESENT
C. RED AND VIOLET LIGHT IS PRESENT
D. SINGLE WAVELENGTH IS PRESENT

- Hint:- In this type of question we have to state the complete definition of the following thing that is asked in the question and had to explain about it. By stating the correct definition we can get our answer.

Complete step-by-step solution -

Monochromatic light, or one-color light, is essentially electromagnetic radiation derived from photon emissions from atoms. Photons propagate, or travel, as energy wave fronts of different lengths and levels of energy.
The term monochromatic derives from the Greek words monos, meaning one or sole, and chromos, meaning color.
Photons propagate, or travel, as energy wave fronts of different lengths and levels of energy. Technologies using monochromatic light have a wide range of applications, from astrophysics and astronomy to forensic science.
A monochromatic light beam is characterized by its brightness or light intensity, direction of propagation, and color (all visible characteristics) and by its state of polarization (an invisible characteristic).
When a monochromatic light is directed to a substance or material, it induces transitions which are characteristic to the chemical properties of the constituent elements of such material.
Thus, we can conclude that answer is D. SINGLE WAVELENGTH IS PRESENT

Note: Such type of questions can only be done by explaining the wholesome definition of the monochromatic lights. As explained the full basics of such definition can help you get a better understanding and Sodium lamp is a common source of monochromatic light.