If $m$ is the A.M of two distinct real numbers $l$ and $n (l ,n > 1) $ and $G_1,G_2$ and $G_3$ are three geometric means between l and n, then $G_1^4 +2 G_2^4+G_3^4 $ equals

Question

$\begin{array}{1 1} 4\;l^2\;mn \\ 4lm^2n \\ 4\;lmn^2 \\ 4l^2m^2n^2 \end{array} $