The correct option is B Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
If the absorptive and emissive powers of a body are not equal, a body would continuously lose or gain heat energy from radiation, which would cause it to continuously get hotter, or cooler.
This doesn't happen; so we can conclude that absorptive and emissive powers are equal for all bodies.
It is true that they are both dimensionless quantities(constants), but that is not the correct explanation.