If $f(x,y )= \large\frac{\cos (x-4y)}{\cos (x+4y)}$, then $\large\frac{\partial f}{\partial x} \bigg|_{y-\large\frac{x}{2}}$ is equal to :

Question

$$\begin {array} {1 1} (a)\;-1 & \quad (b)\;0 \\ (c)\;1 & \quad (d)\;2 \end {array}$$