In the following sequence of reaction, the final product (Z) is

$\mathrm{HC}\equiv \mathrm{CH}\underset{{\mathrm{H}}_2{\mathrm{SO}}_4}{\overset{{\mathrm{Hg}}^{+2}}{\to }} \mathrm{X} \underset{{\mathrm{H}}_2\mathrm{O}}{\overset{{\mathrm{CH}}_3\mathrm{MgX}}{\to }} \mathrm{Y} \stackrel{\left[\mathrm{O}\right]}{\to } \mathrm{Z}$

When we treat ethyne with $H{g}^{+2}/H_{2}S{O}_4$ then product is acetaldehyde(X) after this when we treat X with $C{H}_{3}MgX/H_{2}O$ then propanal(Y) formed at last on oxidation we will get propanone(Z).

 $HC\equiv CH$ $\stackrel{H{g}^{+2}/H_{2}S{O}_4}{\to }$ $C{H}_{3}CHO$$\stackrel{C{H}_{3}MgX/H_2{O}_{ }}{\to }$ $C{H}_3-CH\left(OH\right)-C{H}_3$ $\stackrel{\left(O\right)}{\to }$$C{H}_3-CO-C{H}_3$

Therefore, correct option is: (C) propanone.