(d)

Total photons emitted from source is

$N=\frac{\text { Power }}{h c / \lambda}$

$=\frac{160 \times 50000 \times 10^{-10}}{6.6 \times 10^{-34} \times 3 \times 10^8}$

So, photon flux falling over spherical surface of radius $r=18\,m$ is

$n=\frac{N}{A}=\frac{N}{4 \pi r^2}$

$=\frac{160 \times 50000 \times 10^{-6}}{6.6 \times 10^{-34} \times 3 \times 10^8 \times 4 \pi \times(18)^2}$

$\approx 10^{20} \,m ^{-2} s ^{-1}$