Which of the following gives white precipitate with ammonical silver nitrate…

Ammoniacal silver nitrate solution (Tollens'-type reagent, [Ag(NH₃)₂]NO₃) reacts specifically with terminal alkynes - hydrocarbons containing a carbon-carbon triple bond with at least one hydrogen atom attached directly to a triple-bonded (sp-hybridised) carbon. Because of the high s-character of the sp carbon, this hydrogen is weakly acidic and is readily displaced by Ag⁺ ions, forming an insoluble white precipitate of a silver acetylide (a metal alkynide). Alkanes and alkenes contain no such acidic hydrogen, so they do not react with this reagent.

Among the given compounds, Acetylene (HC≡CH, C₂H₂) is the only one with a carbon-carbon triple bond, and both of its terminal carbons carry a hydrogen atom attached directly to an sp carbon. These hydrogens are displaced by Ag⁺ from the ammoniacal silver nitrate solution, producing disilver acetylide (Ag-C≡C-Ag), which separates out as a white precipitate.

• Ethane (C₂H₆): has only C-C and C-H single bonds (sp³ carbons); with no unsaturation and no acidic hydrogen, it does not react with ammoniacal silver nitrate.

• Ethylene (C₂H₄): has a carbon-carbon double bond (sp² carbons), but its vinylic hydrogens are not acidic enough to be displaced by Ag⁺, so alkenes give no precipitate with this test.

• Propene (C₃H₆): also has only a carbon-carbon double bond (sp² carbons) and no triple bond; like ethylene, it does not react with ammoniacal silver nitrate.

This white-precipitate test with ammoniacal silver nitrate identifies a terminal alkyne by its acidic, triple-bond-adjacent hydrogen. Acetylene's structure meets that requirement, so its terminal hydrogens are the ones displaced by Ag⁺ to form the white silver-acetylide precipitate.