Answer:
The line of reflection is y.
Explanation:
If it was reflection by the x axis, it would of been in the same position, just flipped around.
If it was reflection by the z axis, the two shapes would of been much closer (or further, depends how you look at it) than it is in graph.
Therefore, it is reflected by the y axis. You can also tell since both shapes are equidistant from the y axis.
Hope this helps!