The threshold of hearing is

and in order to hear the howler at a distance of 5 km, its sound should have at least this intensity.
The sound wave is emitted uniformly in all directions, so the area we should consider to calculate the power is the area of a sphere with radius
r=5 km=5000 m:

And by using these data, we can calculate the power emitted by the howler:

So, the correct answer is E) 0.31 mW.