An alkaline earth metal (M) dissolves in liquid ammonia forming a deep blue-black solution. The reaction involved is

• A

  $\mathrm{M}+2 \mathrm{NH}_3 \rightarrow \mathrm{M}\left(\mathrm{NH}_2\right)_2+\mathrm{H}_2$
• B

  $\mathrm{M}+x \mathrm{NH}_3 \rightarrow\left[\mathrm{M}\left(\mathrm{NH}_3\right)_x\right]$
• C

  $\mathrm{M}+2 \mathrm{NH}_4 \mathrm{OH} \rightarrow \mathrm{M}(\mathrm{OH})_2+2 \mathrm{NH}_3$
• Correct answer
  D

  $\mathrm{M}+(x+2 y) \mathrm{NH}_3 \rightarrow\left[\mathrm{M}\left(\mathrm{NH}_3\right)_x\right]^{2+}+2\left[\mathrm{e}\left(\mathrm{NH}_3\right)_y\right]^{-}$. (e represents electron)

Hence, the correct answer is option: 'D'.