The correct answer is: An optically plane surface reflects a beam of light as a parallel beam in one direction.
A plane mirror is a flat mirror that reflects light in a single direction. This means that if you shine a beam of light at a plane mirror, the light will reflect off the mirror and travel in a straight line in the opposite direction.
A diffused beam is a beam of light that is scattered in all directions. This can happen when light hits a rough surface, such as a piece of paper. The light waves are reflected off the surface in different directions, and this creates a diffused beam.
A parallel beam is a beam of light that travels in a straight line and does not spread out. This can happen when light hits a smooth surface, such as a mirror. The light waves are reflected off the surface in the same direction, and this creates a parallel beam.
Therefore, the correct answer is: An optically plane surface reflects a beam of light as a parallel beam in one direction.