Answer : 3. "Plane Mirror "
Explanation :

1. Which plane mirror is used to make periscope?

2. An optical instrument used to see through an object that is occluded from the direct line of sight is called a periscope.

3. A periscope consists of two plane mirrors placed at an angle of 45°.

4. When light falls on one of the mirrors then after reflection from the first mirror it falls on the second mirror which further reflects it and falls on the eye of the observer.