Final answer:
The transformation is a reflection over the x-axis, as indicated by the unchanged x-coordinate and the inverted y-coordinate.
Step-by-step explanation:
When point E is located at (-3,1) and point E' is located at (-3,-1), the transformation that has occurred is a reflection over the x-axis. This can be determined because the x-coordinate stays the same, while the y-coordinate is the same number but with an opposite sign. Reflection over the x-axis implies that every point of the original figure is flipped across the x-axis to a position that is equally far away on the opposite side of the axis.