Correct Answer - Option 2 : 45°
Explanation:
Optical square and Prism square are instruments used to set out a right angle to a chain line.
(i) Optical Square: It is more convenient and accurate than cross-staff for setting out right angles. Consists of two mirrors making a 45° with each other, one mirror totally silvered another top bottom un-silvered.
(ii) Prism Square: It works on the same principle as that of the optical square. It is more modern and precise instruments and has the merit that no adjustment is required since the angle between the reflection surface (i.e. 45°) cannot vary.