A. the plate voltage is zero

B. the plate voltage is slightly negative

C. the plate voltage is slightly positive

D. the temperature of the filament is low

When the temperature of the filament is low, then there is a possibility that not enough kinetic energy is being produced and due to these electrons may not able to leave the surface of the cathode and hence zero current in a diode. When plate voltage is zero it means indirectly that current is zero as we know both are in direct relation to each other. And when the plate’s voltage is negative with respect to cathode no electrons will leave the surface and hence zero current. Now when the voltage is slightly positive, it may be that it is not greater than forward voltage and hence electrons are not able to accelerate towards the anode. As we know around cathode a cloud of electrons formed and when voltage is not enough for electrons, they will not able to reach the anode.