Answer:
A. the perpendicular bisector of the line segment XY
Explanation:
You know that one point equidistant from X and Y is the midpoint of segment XY. The third vertex of an isosceles triangle with XY as its base will also be equidistant from X and Y. That vertex will be located on the perpendicular bisector of XY.
The locus of points of interest is the perpendicular bisector of XY.
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For many geometry problems, it can be helpful to draw a diagram.