ANS:D - double reflection.

Double reflection because it having two mirrors adjusting in 45 degree each other. Double reflection:

1. Index glass.
2. Horizon glass. Optical square consists of two mirrors, one mirror totally silvered another top silvered bottom unsilvered. Based on double reflection the Answer D is correct. The principle is reflection only not a double reflection because it works by the reflection of light, does not matter it is one time or two times, reflection happens that's only the principle. Optical square based on principal of reflection but the construction of optical square is based on double reflection. Measured angle-Reflection.
Principle- Double Reflection.