The number of photo-electrons emitted per second from a metal surface increases when:

The correct curve between the stopping potential $(V_0)$ and intensity of incident light $(I)$ is

Light of frequency two times the threshold frequency is incident on a photosensitive material. If the incident frequency is made $\left(\frac{1}{3}\right)^{rd}$ and intensity is doubled,then the photoelectric current will

Consider the two following statements $A$ and $B$, and identify the correct choice given in the answers :
$(A)$ In photovoltaic cells, the photoelectric current produced is not proportional to the intensity of incident light.
$(B)$ In gas-filled photoemissive cells, the velocity of photoelectrons depends on the wavelength of the incident radiation.

The wave picture of light has failed to explain which of the following phenomena?

Which of the following phenomena can explain the quantum nature of light?