Answer:
B. across the y-axis
Step-by-step explanation:
The line of reflection is the perpendicular bisector of the segment between the orginal point and its image. If you plot the two points, you see that the y-axis is that bisector.
The point was reflected across the y-axis.
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Additional comments
Changing the sign of the x-coordinate means a point that was some distance on one side of the y-axis is now that same distance on the other side. That is, the y-axis is the line of reflection when the sign of x changes.
Similarly, the x-axis is the line of reflection when the sign of y changes.
(x, y) ⇒ (-x, y) . . . . reflection across the y-axis
(x, y) ⇒ (x, -y) . . . . reflection across the x-axis
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Changing the signs of both coordinates (in either order) is effectively a reflection across the origin. There are other names for this reflection, too.
(x, y) ⇒ (-x, -y) . . . . reflection across both axes, across the orign, rotation 180°
