Final answer:
Brielle applied a reflection across the y-axis to point X (12,5), resulting in the transformation of the point to (-12,5).
Step-by-step explanation:
The transformation rule Brielle applied to point X (12,5), given by the rule (x,y) to (-x,y), results in the coordinates of point X changing from (12,5) to (-12,5). This rule changes the x-coordinate to its opposite value while keeping the y-coordinate the same. The way that this rule alters the x-coordinate is consistent with a reflection across the y-axis, since all points are flipped over the y-axis to their mirror position in terms of the x-coordinate while their y-coordinate remains unchanged.
Thus, the correct answer to the transformation applied to point X would be option A) Reflection across the y-axis.