The blue colour of the solution (metal dissolved in liquid ammonia) is due to:

The blue colour of the solution (metal dissolved in liquid ammonia) is due to ammoniated ${\mathrm{e}}^{-}$.

Ammoniated ${\mathrm{e}}^{-}$ absorbs energy in the visible region of light and thus imparts blue color to the solution.

$M+(x+y){\mathrm{NH}}_3\to {\left[M{\left({\mathrm{NH}}_3\right)}_x\right]}^{+}+\left[e{\left({\mathrm{NH}}_3\right)}_y\right]$

$M_{\text{(am) }}^{+}+e^{-}+{\mathrm{NH}}_3\left(l\right)\to M{\mathrm{NH}}_{2\left(\mathrm{am}\right)}+\frac{1}{2}{\mathrm{H}}_2\left(g\right)$

Hence, option C is correct.