a
According to the Stefan Boltzmann law, the power radiated by the star whose surface radiates as a black body at temperature \(T\,K\) is given by

\(P = \sigma 4\pi {r^2}{T^4}\)

Where, \(r=radius\,of\,the\,star\)

\(\sigma=Stefan's\,constant\)

The radiant power per unit area received at a distance \(R\) from the centre of a star is

 \(S = \frac{P}{{4\pi {R^2}}} = \frac{{\sigma 4\pi {r^2}{T^4}}}{{4\pi {R^2}}} = \frac{{\sigma {r^2}{T^4}}}{{{R^2}}}\)