A corner reflector is converted to plane reflector when the corner

A corner reflector is converted to plane reflector when the corner angle is ______
A. 90°
B. 180°
C. 45°
D. 60°

Right option is B. 180°

For a corner reflector to become a plane reflector the corner angle must be 180°.
For a plane reflector N (current element) =1
$$α=\frac{π}{N}=π.$$