Initially, when the voltage of the collecter is , there is a current measured in the ammeter assuming that the photon energy is greater than the work function . As the voltage is lowered, the electrons are decelerated by the electric field between the photocathode and the collector. So, when the electrons reach the collector, they have a smaller velocity and so the measured current is 0. Eventually, when the voltage is lowered enough, the electrons reaching the collector have a velocity of exactly , so no current is measured. This occurs when the given equation is satisfied. Therefore, answer (A) is correct.
