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 $T\text{ K}$ is given by:

1. A
   $\frac{r^2\sigma T^4}{4\pi R^2}$
2. B
   $\frac{r^2\sigma T^4}{R^2}$
3. C
   $\frac{4\pi r^2\sigma T^4}{r^2}$
4. D
   $\frac{r^4\sigma T^4}{R^4}$