Final answer:
The correct coordinates for a vertex of the reflected triangle A'B'C' are D. (-4, 5), as reflection over the y-axis changes the sign of the x-coordinate while keeping the y-coordinate unchanged.
Step-by-step explanation:
When reflecting a triangle across the y-axis, each vertex of the triangle will have its x-coordinate change sign, while the y-coordinate remains the same. Therefore, if we have a vertex of a triangle with coordinates (x, y), the mirrored vertex in triangle A'B'C' will have coordinates (-x, y). Given the choices provided, we must look for a point that is the reflection of a point on the y-axis. The correct coordinates for a vertex of triangle A'B'C' after reflection would be the option where the original x-coordinate is negated.
The correct answer for the reflected vertex is D. (-4, 5) because the reflection of the point along the y-axis only changes the x-coordinate. If a point had an original coordinate of (4, 5), reflecting it over the y-axis would yield (-4, 5).