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

$x'z + xz'=x\oplus z.$

Option B.