Reflection over the y-axis.
1) Let's start by locating each coordinate from the Pre-image and the Image:
Pre-image Image
A (1,1) A'(-1,1)
B (5,1) B' (-5,1)
C (4,4) C' (-4,4)
2) As each coordinate from the Image has the x-coordinate opposite to the Pre-image's x-coordinate we can state that the transformation performed was:
Reflection over the y-axis.