A figure that is reflected in the axis would have its image directly below or above it on the opposite side of that axis. By the same reasoning, a figure reflected in the axis, would have an image to the left or right of it on the opposite side of that axis. With this in mind, let's consider the given diagram.
We see above that the triangles are neither directly above nor below each other. Therefore, the transformation shown is not a reflection in the axis. We can also see that the triangles are not directly next to each other. Hence, the transformation is not a reflection in the axis. The answer is neither.