In the nitroprusside ion,the iron exists as $Fe^{2+}$ and $NO$ as $NO^{+}$ rather than $Fe^{3+}$ and $NO$ respectively. These forms of ions are established with the help of

• A
  magnetic moment in solid state
• B
  thermal decomposition method
• C
  by reaction with $KCN$
• D
  by action with $K_2SO_4$