ANS:C - 45°

In surveying optical square is used to setting out right angles. The horizon glass is placed at an angle of 120° with the horizon sight and index glass is placed at an angle of 105° with the index sight. In the SINGLE optical square, it contains two reflective planes inside, and the angle between two planes is 45°,

Just draw these 4 angles of optical square on a paper, you will understand it easily 45°, 90°, 112.5 °, 112.5°, (the two planes are at 45° inside the optical square). When we keep two mirror at an angle 45°, it will make angle of 90° by double mirror reflection.