Reaction involved: hydration of alkynes in presence of $HgSO_4/H_2SO_4$

In acidic mercuric sulfate catalyzed hydration, water adds across the triple bond of an alkyne to first give an enol, which then undergoes keto-enol tautomerism to give a carbonyl compound.

Products formed from different alkynes

• For a terminal alkyne $R{-}C\equiv CH$: $R{-}C\equiv CH \xrightarrow[H_2SO_4]{HgSO_4,\ H_2O} R{-}CO{-}CH_3$ So terminal alkynes give methyl ketones.

• For acetylene $HC\equiv CH$: $HC\equiv CH \xrightarrow[H_2SO_4]{HgSO_4,\ H_2O} CH_3CHO$ This gives ethanal.

• For an internal alkyne: hydration gives a ketone (or mixture of ketones if unsymmetrical).

Important conclusion

By this method, the obtainable carbonyl compounds are:

• ketones
• ethanal as the only aldehyde obtainable from acetylene

Therefore, any aldehyde other than ethanal cannot be prepared by this method.

Hence

Among the given options, the correct one must be the compound corresponding to an aldehyde other than ethanal, which is Option C.

Comparison with stored answer

Stored correct answer = C

Our derived answer = C

So they agree.