Assuming the Sun to be a perfectly spherical black body of radius R...

The total power radiated by the Sun is given by Stefan's law:

$P = \sigma A T^4 = \sigma (4\pi R^2) T^4$

The intensity of this radiation at a distance $r$ from the Sun is:

$I = \dfrac{P}{4\pi r^2} = \dfrac{\sigma (4\pi R^2) T^4}{4\pi r^2} = \dfrac{\sigma R^2 T^4}{r^2}$

The Earth intercepts this radiation over its cross-sectional area, which is $\pi R_o^2$.

The total radiant power incident on the Earth is:

$P_{incident} = I \times \pi R_o^2 = \dfrac{\pi R^2 R_o^2 \sigma T^4}{r^2}$

Answer: $\dfrac{\pi R^2 R_o^2 \sigma T^4}{r^2}$