The anode voltage of a photocell is kept fixed. The wavelength $$\lambda$$ of the light falling on the cathode is gradually changed. The plate current I of the photocell varies as follows :

$$E = \frac{hc}{\lambda}$$

$$\lambda \leq \lambda_0 \text{ where } \lambda_0 = \frac{hc}{\phi}$$ ($$\lambda_0$$ is threshold wavelength)

As the wavelength $$\lambda$$ increases, the energy per photon decreases. Thus, the maximum kinetic energy of emitted electrons ($$K_{max} = \frac{hc}{\lambda} - \phi$$) decreases. With lower kinetic energy and a fixed anode voltage, fewer electrons have sufficient energy to overcome the space charge or reach the anode, causing the plate current $$I$$ to decrease.

Once $$\lambda$$ exceeds the threshold wavelength $$\lambda_0$$, photon energy becomes insufficient to eject electrons. At this point, the plate current $$I$$ drops to zero. 

All these are correctly shown in the graph in Option (B).