The correct answer is Option 4: Nickel and H2.

Explanation

The reduction of an unsaturated hydrocarbon (like an alkene or alkyne) to a saturated hydrocarbon (an alkane) is called hydrogenation. This process involves adding hydrogen (\(\text{H}_2\)) across the carbon-carbon double or triple bonds.

Because the reaction has a high activation energy, it requires a transition metal catalyst to proceed efficiently. Nickel (Ni), along with other noble metals like Palladium (Pd) or Platinum (Pt), provides a surface that adsorbs both the hydrogen molecules and the unsaturated hydrocarbon, weakening their bonds and allowing the reaction to happen smoothly.

A classic example is the hydrogenation of ethene to ethane using a nickel catalyst at elevated temperatures (around 150°C to 300°C), a process historically known as the Sabatier-Senderens reduction:

\(\text{CH}_2=\text{CH}_2 + \text{H}_2 \xrightarrow{\text{Ni, }\Delta} \text{CH}_3-\text{CH}_3\)

This catalytic system is also widely used in the food industry to convert liquid vegetable oils (unsaturated fats) into solid vanaspati ghee or margarine (saturated fats).

Why the other options are incorrect:

Copper (Cu), Iron (Fe), and Zinc (Zn) are not effective catalysts for the direct addition of molecular hydrogen (\(\text{H}_2\)) across carbon-carbon double bonds under standard hydrogenation conditions. Iron is traditionally used in the Haber process for synthesizing ammonia, while zinc is often used with acid (\(\text{Zn/HCl}\)) for different types of reductions, such as reducing carbonyl groups or alkyl halides via nascent hydrogen.