Which one of the following is used in making periscope?

Plane mirrors (C) are used in making periscopes. A periscope typically consists of two plane mirrors positioned parallel to each other at 45-degree angles to the line of sight. This arrangement allows light entering the periscope from one end to be reflected off the first mirror down the length of the periscope to the second mirror, and then out of the periscope to the viewer’s eye. This design enables the user to see over or around obstacles, making it useful in submarines, tanks, and for observing events from concealed positions. The use of plane mirrors is essential because they reflect light without altering its divergence, unlike concave or convex mirrors which would distort the image. This setup provides a clear, upright image to the observer, facilitating effective surveillance and observation.