An organic compound X gives a red precipitate on heating with Fehling's solution. The reaction that yields X as major product is 

The compound X gives a red precipitate on heating with Fehling's solution.
This indicates that the compound X is an aldehyde.
It can be obtained by the acid-catalyzed hydration of acetylene.
When acetylene reacts with water in the presence of sulphuric acid and mercuric sulphate, vinyl alcohol is obtained.

Vinyl alcohol on keto-enol tautomerism gives acetaldehyde (compound X). 

Hence, the correct answer is option (D)