Answer:
A. When b = 0
Explanations:
The linear function is:
f(x) = mx + b
f(kx) means that we should replace x by kx anywhere we see it in the function.
Mathematically,
f(kx) = m(kx) + b
f(kx) = mkx + b
kf(x) means that we should multiply the function f(x) by k
Mathematically,
k f(x) = k(mx + b)
k f(x) = mkx + kb
If b = 0:
f(kx) = mkx + 0
f(kx) = mkx
kf(x) = mkx + k(0)
kf(x) = mkx + 0
kf(x) = mkx
Therefore, kf(x) = f(kx) when b = 0