ANS:A - emissivity independent of wavelength.

In radiative heat transfer, a grey surface is the one which has emissivity independent of wavelength. A grey surface, in the context of radiative heat transfer, is defined as a surface that emits and absorbs radiation uniformly across all wavelengths. In other words, its emissivity (which quantifies how efficiently a surface emits radiation compared to a blackbody at the same temperature) is constant and does not vary with the wavelength of the radiation. The other options provided do not accurately define a grey surface:

• Grey appearance to the eye: This describes the visual perception of a surface, which may or may not correlate with its radiative properties.
• Zero reflectivity: A grey surface may reflect some radiation, but its reflectivity is not necessarily zero. Reflectivity depends on factors such as surface roughness, material properties, and the angle of incidence.
• Equal brightness in all directions: This describes a diffuse reflector, where the brightness of the reflected light is the same regardless of the viewing angle. While this property is related to the concept of a grey surface, it does not fully capture the radiative properties of such a surface.