Given the function f(x) = x2 and k = –1, which of the following represents a function opening downward?
a. f(x) + k
b. kf(x)
c. f(x + k)
d. f(kx)

To reflect the function across the x axis, we must multiply the whole thing by -1
f(kx) is the answer to make the thing negative
so you should result with
f(x)=-x^2
basically the answer is D

The first one minimumi hope my answer you.