Since we are reflecting the point across the line m, and assuming the line is a proper line (this means that it goes on in both of its sides) then the segment AA' will be the perpendicular bisector of the line.
A situation of this is shown below:
in this graph we have the reflection of a point across the line, and when we do this we end up with a segment that cuts the line in half and that it it perpendicular to the original line.
Therefore the answer is B.
