ANS:C - both (a) and (b)

Kirchhoff's law is applicable to both monochromatic radiation only and total radiation only. Kirchhoff's law states that the ratio of the emissive power to the absorptive power of any surface at a given temperature and wavelength is equal to the emissive power of a blackbody at the same temperature and wavelength.

1. For monochromatic radiation only: Kirchhoff's law applies to the ratio of emissivity to absorptivity for a surface for radiation at a specific wavelength or frequency.
2. For total radiation only: Kirchhoff's law also applies to the ratio of total emissivity to total absorptivity for a surface, which considers all wavelengths of radiation across the electromagnetic spectrum.