The organic compound, which gives a red precipitate with ammoniacal cuprous chloride and undergoes an addition reaction is:

• (A) ethane
• (B) ethene
• (C) ethyne
• (D) ethanol

The organic compound that gives a red precipitate with ammoniacal cuprous chloride and undergoes an addition reaction is ethyne (C).

Explanation of the Reaction

Ammoniacal cuprous chloride is commonly used to test for terminal alkynes. When ethyne reacts with this reagent, it forms a red precipitate, indicating the presence of the alkyne functional group.

Characteristics of the Compounds

• Ethane: A saturated hydrocarbon, does not react with ammoniacal cuprous chloride.
• Ethene: An alkene, does not produce a red precipitate with the reagent.
• Ethyne: A terminal alkyne, reacts to form a red precipitate.
• Ethanol: An alcohol, does not undergo this reaction.

Thus, the correct answer is ethyne, as it meets both criteria of forming a red precipitate and undergoing an addition reaction.