(c) Sulphur trioxide gas
Explanation :
In the contact process, Vanadium pentoxide is used as a catalytic oxidation. It converts the mixture of sulphur dioxide and oxygen to sulphur trioxide.
$2{\mathrm{SO}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\underset{{450}^{\circ }\mathrm{C}}{\overset{{\mathrm{V}}_2{\mathrm{O}}_5}{\mathit{─}\mathit{─}\mathit{─}\mathit{─}\mathit{─}▸}}2{\mathrm{SO}}_3$