Assertion: Photoelectric effect demonstrates the wave nature of light.
Reason: The number of photoelectrons is proportional to the frequency of light.
A. Both A and R are true and R is the correct explanation of A
B. Both A and R are true but R is not correct explanation of A
C. A is true but R is false
D. A and R are false

Hint: Think about the experiment of photoelectric effect. Think about how a photoelectric experiment works and the physics behind it. Then think about the relation between the frequency of light and number of electrons.

Complete step by step solution:
To solve this question, we must know about some pages of the history of science. Let's start. Before 1900, scientists only understood electromagnetic radiation to be made up of waves. So, they also considered light as waves. After that world famous scientist Sir Albert Einstein did the photoelectric effect experiment. Then we all came up In Front of a new concept that is wave-particle duality of light which simply means light is a wave as well as a particle.

Because in this experiment photons of light collide with mobile electrons of metals and due to collision electrons get some kinetic energy. As we increase the frequency of light the energy of photons increases as well as kinetic energy of electrons increases. after gaining a particular amount of energy from the collision after a particular frequency electron comes up from the metal surface. Now here we think if light is a wave then it is not possible because collision between a particle and wave never happens. So light acts as a particle. Therefore, the assertion is wrong.

Now come to the reason. Number of photoelectrons does not increase as the frequency of light increases. Actually, the number of electrons is increased by increasing the intensity of light and increasing frequency do not affect the number of photoelectrons. Another thing is the frequency after which photoelectron stars come out from the metal surface is called threshold frequency. Therefore, we get that the assertion and the reason both are wrong.

Hence the correct option is D.

Note: If light is a wave then it is not possible because collision between a particle and wave never happens. So the photoelectric effect experiment never works. So, light can act as a particle. Number of photoelectrons emitted is directly proportional to the intensity of light. Energy of photoelectrons is proportional to frequency of light.