The number of conducting electrons $\times$ number of holes = constant

Before doping, the number of conducting electrons = number of holes

After doping, the number of conducting electron is $ 2 \times 10^{23}$

$\therefore 3 \times 10^9 \times 3 \times 10^{19} =2 \times 10^{23} \times x$

$x= 4.5 \times 10^{15}$

Hence d is the correct answer.