ANS:D -

The interchange factor for radiation heat transfer from surface 'x' to surface 'y' in the case of infinite parallel planes with emissivities εx​ and εy​ is given by the view factor, often denoted as Fx→y​. The view factor represents the fraction of radiation leaving surface 'x' that directly reaches surface 'y' without being absorbed or scattered by any other surfaces. It depends on the geometry and orientation of the surfaces. For infinite parallel planes, the view factor Fx→y​ is typically 1, meaning all radiation leaving surface 'x' directly reaches surface 'y' without any obstruction. So, the interchange factor for radiation heat transfer from surface 'x' to surface 'y' in this case is 1.