How many plane mirrors are used in a periscope ? How are they arranged relative to each other ?

The correct option is D Two, parallel to each other.
A periscope is an instrument for observation over, around or through an object, obstacle or condition that prevents direct line-of-sight observation from an observer's current position.
A simple periscope is just a long tube with a mirror at each end. The mirrors are fitted into each end of the tube as shown in the figure.
Hence, there are 2 plane mirrors used which are placed parallel to each other.