NaCl reacts with conc. H _ 2 SO _ 4 and K _ 2 Cr _ 2 O _ 7 to give...

Chromyl chloride (B) is produced when a chloride ion reacts with conc.${H}_{2}{SO}_{4}$ and ${K}_{2}{Cr}_{2}{O}_{7}$.
Passing the chromyl chloride gas through $NaOH$ turns the solution yellow. This is due to the formation of ${Na}_{2}{CrO}_{4}$ (C).

The reaction is as follows,

$NaCl + conc . {H}_{2}{SO}_{4} + {K}_{2}{Cr}_{2}{O}_{7} \to \mathop{{CrO}_{2}{Cl}_{2}}_{\mathop{( B )}_{( Reddishbrown )}} + {KHSO}_{4} + {NaHSO}_{4} + {H}_{2}O$

${CrO}_{2}{Cl}_{2} + NaOH \to \mathop{{Na}_{2}{CrO}_{4}}_{\mathop{( C )}_{YellowColour}} + NaCl + {H}_{2}O$