1. Find oxidation state in (A): $Na_4[Fe(CN)_5(NOS)]$

   • The complex ion is $[Fe(CN)_5(NOS)]^{4-}$ because there are $4$ $Na^+$ ions.
   • Each $CN^-$ ligand has charge $-1$, so $5$ cyanides contribute $-5$.
   • The ligand $NOS$ here is nitrosyl sulphide written as $NOS^-$. Its charge is $-1$.
   • Let oxidation state of Fe be $x$.

   Then, $x + 5(-1) + (-1) = -4$ $x - 6 = -4$ $x = +2$

2. Find oxidation state in (B): $Na_4[FeO_4]$

   • The complex ion is $[FeO_4]^{4-}$.
   • Oxygen is $-2$ each, so $4$ oxygens contribute $-8$.
   • Let oxidation state of Fe be $y$.

   Then, $y + 4(-2) = -4$ $y - 8 = -4$ $y = +4$

3. Find oxidation state in (C): $[Fe_2(CO)_9]$

   • Carbon monoxide, $CO$, is a neutral ligand.
   • The complex is overall neutral.
   • Let oxidation state of each Fe be $z$.

   Then, $2z + 9(0) = 0$ $2z = 0$ $z = 0$

4. Calculate the sum:

   $x + y + z = 2 + 4 + 0 = 6$

Therefore, the required sum is: $\boxed{6}$

5. Comparison with stored answer:

   Stored correct answer = $6$

   Our derived answer also = $6$. So they agree.