b
\((a)\) \(\mathrm{CaSO}_{4}+\mathrm{C} \stackrel{\Delta}{\longrightarrow} \mathrm{CaO}+\mathrm{SO}_{2}+\mathrm{CO}\)

\((b)\) \(\mathrm{Fe}_{2}\left(\mathrm{SO}_{4}\right)_{3} \stackrel{\Delta}{\rightarrow} \mathrm{Fe}_{2} \mathrm{O}_{3}+3 \mathrm{SO}_{3}\)

\((c)\) \(S+2 H_{2} S O_{4} \stackrel{\Delta}{\rightarrow} 3 S O_{2}+2 H_{2}O\)

\((d)\) \(H_{2} S O_{4}+P Cl_{5} \stackrel{\Delta}{\longrightarrow} S O_{3} H C I\) Chloro sulphonic acid \(+P O C l_{3}+\mathrm{HCl}\)

Thus, \(S O_{3}\) is obtained by heating \(F e_{2}\left(S O_{4}\right)_{3}\)