The oxidation states of iron atoms in compounds (A), (B) and (C), respectively,

(A) $\mathrm{Na}_{4}\left[\mathrm{Fe}(\mathrm{CN})_{5}(\mathrm{NOS})\right]$

$(+1) 4+x+(-1) 5+(-1) 1=0$

$x=+2$

(B) $\mathrm{Na}_{4}\left[\mathrm{Fe} \mathrm{O}_{4}\right]$

$(+1) 4+y+(-2) 4=0$

$y=+4$

(C) $\left[\mathrm{Fe}_{2}^{\mathrm{z}}(\mathrm{CO})_{9}\right]$

$2 z+0 \times 9=0$

$Z=0$

so $(x+y+z)=+2+4+0$

$=6$