If $ax^2+2hxy+by^2+2gx+2fy+c=0$ represents a pair of parallel lines then $\sqrt {\large\frac{g^2-ac}{f^2-bc}}=$

Question

\[\begin {array} {1 1} (a)\;\frac{a}{b} & \quad (b)\;\sqrt {\frac{a}{b}} \\ (c)\;\sqrt{\frac{b}{a}} & \quad (d)\;\frac{b}{a} \end {array}\]