If the line $lx+my=1$ is normal to the hyperbola

Question

\[\begin {array} {1 1} (a)\;a^2-b^2 & \quad (b)\;a^2+b^2 \\ (c)\;(a^2+b^2)^2 & \quad (d)\;(a^2-b^2)^2 \end {array}\]