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 TK is given by – (Where $\sigma$ is Stefan's Contant)

Question

( A ) $\sigma r^2 \frac{T^4}{R^2}$
( B ) $\sigma r^2 \frac{T^4}{4\pi r^2}$
( C ) $\sigma r^4 \frac{T^4}{ r^4}$
( D ) $4\pi \sigma r^2 \frac{T^4}{R^2}$