ANSWER.
f(x) = - |x| is the correct answer.
Explanation:
We are given the function f(x) = |x| and are required to reflect this function across x- axis.
Now, reflection of a graph across x- axis means we are taking mirror image of the function across x- axis.
Moreover, mirror image does affect the behaviour of the function just the values of the function are negated i.e. f(x) becomes -f(x).
Hence, the reflection of f(x) = |x| is f(x) = -|x|.
It can also be seen in the figure below.