Which does not help in the working of a periscope?

The correct option is A Shadow

A periscope consists of a tube bent at certain points with a plane mirror fixed at certain angles at each bent. When light from the object enters one end of the periscope, it is reflected according to the position and inclination of each mirror. And finally, the light reaches the eye of the observer. So, for the working of a periscope, mirrors, reflection, and light are essential. Shadows are nowhere involved in the process and hence do not help in its working.