So,
We could represent a reflection of a function f(x) using the following facts:
If the transformation of the function f(x) has the form f(-x), it represents a reflection about the y-axis, as you can see in the picture.
If the transformation of the function f(x) has the form -f(x), it represents a reflection about the x-axis, as you can see in the picture.
Then, the graph of y=f(-x) may be obtained by a reflection about the y-axis of the graph of the function f(x).
Therefore, the answer is y.