Main Answer:
The bolded line is a "Perpendicular bisector".
Step-by-step explanation:
A perpendicular bisector is a line that passes through the midpoint of a segment and is perpendicular to that segment.
This line divides the segment into two equal parts. If the bolded line is both perpendicular to the segment and passes through its midpoint, then it is a perpendicular bisector.
Therefore, the correct option is A. Perpendicular bisector.