Which of the following catalytic systems is used for the reduction of unsaturated hydrocarbon to saturated hydrocarbon?

The correct answer is D, Nickel and H2. Adding hydrogen across the double or triple bond of an alkene or alkyne turns it into a saturated alkane; this reaction is called hydrogenation and it needs a catalyst. Finely divided nickel is the standard catalyst, while palladium and platinum also work. The same reaction is used in industry to turn liquid vegetable oils, which have many C=C bonds, into solid vanaspati ghee. In the Sabatier–Senderens reaction, ethene and hydrogen passed over heated nickel give ethane. Option A is wrong because copper is used for dehydrogenation of alcohols and in other oxidation steps, not for hydrogenating oils. Option B is wrong because iron is the catalyst of the Haber process that makes ammonia from nitrogen and hydrogen. Option C is wrong because zinc is a reducing metal used with acids, not a hydrogenation catalyst. Exam tip: nickel = hydrogenation of oils (vanaspati); iron = Haber process; V2O5 = contact process.