Consider the following Boolean function of four variables:
$$f(w, x, y, z) = \Sigma(1, 3, 4, 6, 9, 11, 12, 14)$$
The function is
independent of one variables.
independent of two variables.
independent of three variables.
dependent on all variables
The K-map would be
So, the minimized expression would be
$x'z + xz'=x\oplus z.$