ANS:A - Kirchoffs

The statement "The ratio of the total emissive power to the absorptivity for all bodies is same at thermal equilibrium" represents Kirchhoff's law. Explanation:

• Kirchhoff's law of thermal radiation states that at thermal equilibrium, the ratio of the emissive power (or emissivity) of a body to its absorptivity is the same for all bodies at a given temperature and wavelength.
• Mathematically, it can be expressed as: Emissive powerAbsorptivity=constantAbsorptivityEmissive power​=constant
• This law implies that bodies that are good absorbers of radiation are also good emitters, and vice versa, at a given wavelength and temperature.
• Kirchhoff's law is fundamental in understanding the relationship between emission and absorption of radiation in thermal equilibrium and plays a crucial role in various fields, including thermodynamics, astrophysics, and engineering.
• It is distinct from other laws related to thermal radiation, such as Planck's law, Wien's displacement law, and Stefan-Boltzmann law, which describe different aspects of blackbody radiation and thermal energy transfer.