In the nitroprusside ion,iron and $NO$ exist as $Fe^{II}$ and $NO^{+}$ instead of $Fe^{III}$ and $NO$. Which method is used to confirm this?

• A
  Measuring the concentration of iron
• B
  Measuring the concentration of $CN^{-}$
• C
  Measuring the magnetic moment in the solid state
• D
  Thermally decomposing the compound