Final answer:
The rule to describe a reflection across the y-axis is (x, y) → (-x, y), where the x-coordinate is negated and the y-coordinate remains the same.
Step-by-step explanation:
A reflection across the y-axis can be described by the rule (x, y) → (-x, y), which is option a. In this transformation, the x-coordinate is negated while the y-coordinate remains the same.
For example, if we reflect the point (2, 3) across the y-axis, the resulting point would be (-2, 3).