${\mathrm{C}}_2{\mathrm{H}}_2\stackrel{\text{ Red hot }}{\underset{\text{ metal tube }}{⟶}}\mathrm{A}\stackrel{\text{ Fuming }{\mathrm{H}}_2{\mathrm{SO}}_4}{⟶}\mathrm{B}\text{. }$ B is

 First, benzene forms when ${\mathrm{C}}_2{\mathrm{H}}_2$ reacts with red hot metal tube, and then, after being sulfonated, benzene sulphonic acid is created. The most basic aromatic sulfonic acid and an organosulfur chemical is benzene sulfonic acid. The reaction is as follows: ${\mathrm{C}}_2{\mathrm{H}}_2\underset{500°\mathrm{C}}{\overset{\mathrm{Cu}}{\to }}{\mathrm{C}}_6{\mathrm{H}}_6\underset{\mathrm{fuming}}{\overset{{\mathrm{H}}_2{\mathrm{SO}}_4}{\to }}{\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{SO}}_3\mathrm{H}$

Hence option(C) Benzene sulphonic acid is correct.