An imaginary line perpendicular to a reflecting surface is called a normal.
In geometry, we define the normal of a surface as a vector or a ray that is perpendicular to the surface. (Or a vector that is orthogonal to all the tangent lines to the surface)
The normal can have different lengths, it can be a unit vector (of module 1) or a general vector, so the only thing that defines a normal vector is its perpendicularity to a given surface.
Thus, the imaginary line perpendicular to a reflecting surface is called a normal. Thus the correct option is b.
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