Carbon dioxide and sulphur trioxide cannot undergo oxidation. Potassium dichromate is reduced to chromic sulphate in the presence of sulphur dioxide. The orange solution of potassium dichromate becomes green colour chromic sulphate.

${\mathrm{K}}_2{\mathrm{Cr}}_2{\mathrm{O}}_7 + 3{\mathrm{SO}}_2 + {\mathrm{H}}_2{\mathrm{SO}}_4 \to {\mathrm{K}}_2{\mathrm{SO}}_4 + {\mathrm{Cr}}_2({\mathrm{SO}}_4{)}_3 + {\mathrm{H}}_2\mathrm{O}$

Hence, the gas is sulphur dioxide.