$\tan h^{-1}x = a\log \bigg(\large\frac{1+x}{1-x}\bigg),| x | < 1 =>a=$

Question

\[\begin {array} {1 1} (1)\;1 & \quad (2)\;2 \\ (3)\;\frac{1}{2} & \quad (4)\;\frac{1}{4} \end {array}\]