Answer:
Reflection across the y-axis.
Explanation:
Transformations
The image shows a graph with two shapes ABC and A'B'C'. The coordinates of the vertices of ABC are A(-2,4) B(-1,1) C(1,3).
The coordinates of the vertices of A'B'C' are A'(2,4) B'(1,1) C'(-1,3).
It's clear the points ABC and ABC have the same y-coordinate, but the x-coordinate is inverted, or changed signs.
The transformation that maps a point (x,y) to (-x,y) is a reflection across the y-axis.