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

Kirchhoff's law applies to both total and monochromatic radiation. Kirchhoff's law of thermal radiation states that for a body in thermodynamic equilibrium, the ratio of the emissivity to the absorptivity at a given wavelength and temperature is unity. This law applies to both total radiation (which encompasses all wavelengths) and monochromatic radiation (which refers to radiation at a single specific wavelength). Therefore, the correct option is: "both (a) & (b)."