The line among the following which touches the parabola $y^2=4ax,$ is :

Question

$$\begin {array} {1 1} (a)\;x+my+am^3=0 & \quad (b)\;x-my+am^2=0 \\ (c)\;x+my-am^2=0 & \quad (d)\;x+mx+am^2=0 \end {array}$$