The number of photoelectrons emitted varies with which physical quantity?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A :$ In photoelectric effect,on increasing the intensity of incident light,the stopping potential increases.
Reason $R :$ Increase in intensity of light increases the rate of photoelectrons emitted,provided the frequency of incident light is greater than threshold frequency.
In the light of the above statements,choose the correct answer from the options given below:

Explain Hertz's observation in short.

In a dark room of photography,generally red light is used. The reason is

The quantum nature of light is explained by which of the following phenomena?

Which graph represents the relationship between the intensity of incident light on a metal plate and the resulting photoelectric current?