A ray is reflected in turn by three plain mirrors mutually at right angles to each other. The angle between the incident and the reflected rays is

The angle between the reflected ray and incident ray

1. When the ray of light gets reflected by three mutually perpendicular mirrors, the ray moves in the antiparallel direction.
2. So, the angle between reflected and incident ray is$180°$.

Hence, the angle between reflected and incident ray is$180°$.