ANS:C - under thermal equilibrium.

The statement "The absorptivity of a body is equal to its emissivity at a particular temperature" is actually correct, but with a clarification: The absorptivity (�α) of a body is indeed equal to its emissivity (�ε) at thermal equilibrium and at a particular temperature. This relationship is known as Kirchhoff's law of thermal radiation. Kirchhoff's law states that for a body in thermal equilibrium, the emissivity of the body at a particular wavelength and temperature is equal to its absorptivity at the same wavelength and temperature. In other words, a good emitter of radiation at a particular wavelength is also a good absorber at the same wavelength, and vice versa. So, the correct choice is: c) under thermal equilibrium.