The first step in the construction of a perpendicular bisector is to place the compass point on one endpoint of the line segment and draw an arc that crosses the line segment. Based on your descriptions, the correct response would be:
"A line segment has 2 end points. The left end point is A, and the right is B. A geometric compass is shown with the compass needle on point A and the pencil of the compass placed about two thirds of the way along line segment AB."
This response accurately describes the first step in the construction process.