1. Sulphonation of benzene with oleum

Benzene reacts with oleum to give benzenesulphonic acid:

$\mathrm{C_6H_6 \xrightarrow[ ]{oleum} C_6H_5SO_3H}$

So, compound $X$ is benzenesulphonic acid.

2. Fusion with molten sodium hydroxide

Benzenesulphonic acid on heating with molten NaOH undergoes substitution of the $-\mathrm{SO_3H}$ group by $-\mathrm{OH}$ to form sodium phenoxide, which on acidification gives phenol:

$\mathrm{C_6H_5SO_3H \xrightarrow[molten]{NaOH} C_6H_5ONa \xrightarrow{H^+} C_6H_5OH}$

So, compound $Y$ is phenol.

3. Treatment of phenol with zinc dust/metal

Phenol on heating with zinc dust is reduced to benzene:

$\mathrm{C_6H_5OH + Zn \rightarrow C_6H_6 + ZnO}$

So, compound $Z$ is benzene.

4. Identification of the correct option

Therefore, the structure of $Z$ is benzene.

Hence, the correct option is C.