The total radiant energy per unit area, normal to the direction of incidence, received at a distance R from the center of a star of radius r, whose outer surface radiates as a black body at a temperature T Kelvin is given by:
(Where σ is Stefan's Constant)
A.σr²T⁴/R²
B.σr²T⁴/4πr²
C.σr⁴T⁴/r⁴
D.4πσr²T⁴/R²