24mL concentration of 0.5mol

n(Na2SO3)=0.024L×0.5mol/L=0.012mol, n(K2Cr2O7)=0.02L×0.2mol/L=0.004mol,

Based on the total number of oxidizing and reducing agent valence rises and falls equally, and let the Cr elemental valence be reduced to +x valence, we have:

0.012 mol × (6-4) = 0.004mol × 2 × (6-x),

x=3,

so choose C.