In a diode circuit, for the diode to be forward-biased, the P-end should be connected to a higher potential than the N-end. In option (4), the P-end of the diode is connected to the negative terminal of the power supply, while the N-end is connected to the positive terminal. This configuration causes the diode to be reverse-biased, as it is in opposition to the direction required for forward bias.
Thus the correct answer is option 4.
A pure silicon crystal with 5 × 1028 atoms m−3 has ni = 1.5 × 1016 m−3. It is doped with a concentration of 1 in 105 pentavalent atoms, the number density of holes (per m3) in the doped semiconductor will be: