The total radiant energy per unit area, normal to the direction of incidence, received at a distance R from the centre of a star of radius r, whose outer surface radiates as a black body at a temperature T K is given by :

• A

  $\frac{4{\mathrm{\pi \sigma r}}^2{\mathrm{T}}^4}{{\mathrm{R}}^2}$

• B

  $\frac{{\mathrm{\sigma r}}^2{\mathrm{T}}^4}{{\mathrm{R}}^2}$

• C

  $\frac{{\mathrm{\sigma r}}^2{\mathrm{T}}^4}{4{\mathrm{\pi r}}^2}$

• D

  $\frac{{\mathrm{\pi r}}^4{\mathrm{T}}^4}{{\mathrm{r}}^4}$