Carbon reduction process is used for the extraction of

The Ellingham diagram represents a plot of:

The reactions which occur in a blast furnace at $500-800 \ K$ during the extraction of iron from haematite are:
$(i)$ $3 Fe_2O_3 + CO \rightarrow 2 Fe_3O_4 + CO_2$
$(ii)$ $Fe_2O_3 + 3 C \rightarrow 2 Fe + 3 CO$
$(iii)$ $Fe_3O_4 + 4 CO \rightarrow 3 Fe + 4 CO_2$
$(iv)$ $Fe_2O_3 + CO \rightarrow 2 FeO + CO_2$

$FeCr_2O_4$ (chromite) is converted to $Cr$ by following steps:
Chromite $\xrightarrow{I} Na_2CrO_4$ $\xrightarrow{II} Cr_2O_3$ $\xrightarrow{III} Cr$
Reagents in $I, II$ and $III$ steps might be:
$I$ Step $-$ $II$ Step $-$ $III$ Step

$I$. $FeCr_2O_4 + NaOH + \text{air} \rightarrow (A) + Fe_2O_3$
$II$. $(A) + (B) \rightarrow Na_2Cr_2O_7$
$III$. $Na_2Cr_2O_7 + X \xrightarrow{\Delta} Cr_2O_3$
$IV$. $Cr_2O_3 + Y \xrightarrow{\Delta} Cr$
High temperature $(> 1000 \ ^\circ C)$ electrolytic reduction is necessary for isolating:

Which of the following is an acidic flux?